Issue title: Installation issue: llvm; spack environment; external compiler; bad rpath
 Issue body: ### Steps to reproduce the issue

```console
$ spack install {{EMAIL+EMAIL}}~all_targets build_type=Release +clang+compiler-rt+gold+internal_unwind+libcxx+link_dylib+lld+lldb~omp_tsan+polly~python~shared_libs arch=linux-centos7-haswell
...
../../../../../bin/llvm-tblgen: /usr/lib64/libstdc++.so.6: version `GLIBCXX_3.4.20' not found (required by ../../../../../bin/llvm-tblgen)
../../../../../bin/llvm-tblgen: /usr/lib64/libstdc++.so.6: version `CXXABI_1.3.8' not found (required by ../../../../../bin/llvm-tblgen)
../../../../../bin/llvm-tblgen: /usr/lib64/libstdc++.so.6: version `GLIBCXX_3.4.21' not found (required by ../../../../../bin/llvm-tblgen)
make[2]: *** [tools/clang/include/clang/Driver/Options.inc.tmp] Error 1

```

### Platform and user environment

```commandline
$ uname -a
Linux darwin-fe1.lanl.gov 3.10.0-1062.1.1.el7.x86_64 #1 SMP Fri Sep 13 22:55:44 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
$ lsb_release -d
Description:    CentOS Linux release 7.7.1908 (Core)
``` 

- My relevant `spack.yaml` Environment bits:
```commandline
spack:
  include:
  - ../../../../etc/spack/linux/packages.yaml
  - ../../../../etc/spack/mirrors.yaml
  - ../../../../etc/spack/modules.yaml
  - ../../../../etc/spack/repos.yaml
  specs:
  - llvm
```

- My relevant `../../../../etc/spack/linux/packages.yaml` bits:
```commandline
packages:
  all:
    compiler: ['ekelley@example.net']
  llvm:
    variants: ['+link_dylib']
    version: ['6.0.1']
```
- My `../../../../etc/spack/linux/compilers.yaml`:
```commandline
compilers:
- compiler:
    environment: {}
    extra_rpaths: []
    flags: {}
    modules:
    - 'gcc/9.2.0'
    operating_system: centos7
    paths:
      cc: /projects/opt/x86_64/gcc/9.2.0/bin/gcc
      cxx: /projects/opt/x86_64/gcc/9.2.0/bin/g++
      f77: /projects/opt/x86_64/gcc/9.2.0/bin/gfortran
      fc: /projects/opt/x86_64/gcc/9.2.0/bin/gfortran
    spec: ekelley@example.net
    target: x86_64
```

### Additional information

This is most certainly an rpath issue - llvm is building intermediary binaries to execute during its complete build cycle whose rpaths resolve to a system libstdc++.so before any provided by my configured ekelley@example.net compiler.  Some more details:

```console
$ spack build-env llvm | grep -i rpath
SPACK_CC_RPATH_ARG=-Wl,-rpath,
SPACK_COMPILER_IMPLICIT_RPATHS=/projects/opt/x86_64/gcc/9.2.0/lib/gcc/x86_64-pc-linux-gnu/9.2.0:/projects/opt/x86_64/gcc/9.2.0/lib64
SPACK_CXX_RPATH_ARG=-Wl,-rpath,
SPACK_F77_RPATH_ARG=-Wl,-rpath,
SPACK_FC_RPATH_ARG=-Wl,-rpath,
SPACK_RPATH_DIRS=/projects/shavano/packages/DWX86G/spack/opt/spack/linux-centos7-haswell/gcc-9.2.0/llvm-6.0.1-ubw7yfm762ridzao7vetl5leqzb7doqc/lib:/projects/shavano/packages/DWX86G/spack/opt/spack/linux-centos7-haswell/gcc-9.2.0/llvm-6.0.1-ubw7yfm762ridzao7vetl5leqzb7doqc/lib64:/projects/shavano/packages/DWX86G/spack/opt/spack/linux-centos7-haswell/gcc-9.2.0/pcre-8.42-5l6cyodg32is2knv3cq3xdkfqppl7pyz/lib:/projects/shavano/packages/DWX86G/spack/opt/spack/linux-centos7-haswell/gcc-9.2.0/gettext-116.69.203.115-j7kkyaninjrqyghhaqc2vvjoim4tzwth/lib:/projects/shavano/packages/DWX86G/spack/opt/spack/linux-centos7-haswell/gcc-9.2.0/libedit-3.1-20170329-4y2hnj7l5tmwythhkz2rs2dhvx7c7vet/lib:/projects/shavano/packages/DWX86G/spack/opt/spack/linux-centos7-haswell/gcc-9.2.0/zlib-1.2.11-35samuwqrkyjaoigifegblhqx5idjwse/lib:/projects/shavano/packages/DWX86G/spack/opt/spack/linux-centos7-haswell/gcc-9.2.0/binutils-2.32-p5xuojl4b3ziamr362ct7cccqw4u7vo3/lib
```

```console
$ readelf -d llvm-tblgen | grep -i rpath 
 0x000000000000000f (RPATH)              Library rpath: [/projects/shavano/packages/DWX86G/spack/opt/spack/linux-centos7-haswell/gcc-9.2.0/llvm-6.0.1-ubw7yfm762ridzao7vetl5leqzb7doqc/lib:/projects/shavano/packages/DWX86G/spack/opt/spack/linux-centos7-haswell/gcc-9.2.0/llvm-6.0.1-ubw7yfm762ridzao7vetl5leqzb7doqc/lib64:/projects/shavano/packages/DWX86G/spack/opt/spack/linux-centos7-haswell/gcc-9.2.0/binutils-2.32-p5xuojl4b3ziamr362ct7cccqw4u7vo3/lib:/projects/shavano/packages/DWX86G/spack/opt/spack/linux-centos7-haswell/gcc-9.2.0/gettext-116.69.203.115-j7kkyaninjrqyghhaqc2vvjoim4tzwth/lib:/usr/lib:/projects/shavano/packages/DWX86G/spack/opt/spack/linux-centos7-haswell/gcc-9.2.0/zlib-1.2.11-35samuwqrkyjaoigifegblhqx5idjwse/lib:/projects/shavano/packages/DWX86G/spack/opt/spack/linux-centos7-haswell/gcc-9.2.0/libedit-3.1-20170329-4y2hnj7l5tmwythhkz2rs2dhvx7c7vet/lib:/projects/shavano/packages/DWX86G/spack/opt/spack/linux-centos7-haswell/gcc-9.2.0/pcre-8.42-5l6cyodg32is2knv3cq3xdkfqppl7pyz/lib:/usr/lib64:/projects/opt/x86_64/gcc/9.2.0/lib/gcc/x86_64-pc-linux-gnu/9.2.0:/projects/opt/x86_64/gcc/9.2.0/lib64]
```
(Note: /usr/lib64 comes before my compiler's lib64!!!)
 
You can see that my system does not provide GLIBCXX_3.4.20 and friends (as the error message indicates):

```console
$ strings /usr/lib64/libstdc++.so.6 | grep ^GLIBCXX
GLIBCXX_3.4
GLIBCXX_3.4.1
GLIBCXX_3.4.2
GLIBCXX_3.4.3
GLIBCXX_3.4.4
GLIBCXX_3.4.5
GLIBCXX_3.4.6
GLIBCXX_3.4.7
GLIBCXX_3.4.8
GLIBCXX_3.4.9
GLIBCXX_3.4.10
GLIBCXX_3.4.11
GLIBCXX_3.4.12
GLIBCXX_3.4.13
GLIBCXX_3.4.14
GLIBCXX_3.4.15
GLIBCXX_3.4.16
GLIBCXX_3.4.17
GLIBCXX_3.4.18
GLIBCXX_3.4.19
GLIBCXX_DEBUG_MESSAGE_LENGTH
```

but my compiler's libstdc++.so does:
```console
$ strings /projects/opt/x86_64/gcc/9.2.0/lib64/libstdc++.so.6 | grep ^GLIBCXX
GLIBCXX_3.4
GLIBCXX_3.4.1
GLIBCXX_3.4.2
GLIBCXX_3.4.3
GLIBCXX_3.4.4
GLIBCXX_3.4.5
GLIBCXX_3.4.6
GLIBCXX_3.4.7
GLIBCXX_3.4.8
GLIBCXX_3.4.9
GLIBCXX_3.4.10
GLIBCXX_3.4.11
GLIBCXX_3.4.12
GLIBCXX_3.4.13
GLIBCXX_3.4.14
GLIBCXX_3.4.15
GLIBCXX_3.4.16
GLIBCXX_3.4.17
GLIBCXX_3.4.18
GLIBCXX_3.4.19
GLIBCXX_3.4.20
GLIBCXX_3.4.21
GLIBCXX_3.4.22
GLIBCXX_3.4.23
GLIBCXX_3.4.24
GLIBCXX_3.4.25
GLIBCXX_3.4.26
GLIBCXX_3.4.27
GLIBCXX_DEBUG_MESSAGE_LENGTH
GLIBCXX_3.4.21
GLIBCXX_3.4.9
GLIBCXX_3.4.10
GLIBCXX_3.4.16
GLIBCXX_3.4.1
GLIBCXX_3.4.25
GLIBCXX_3.4.24
GLIBCXX_3.4.20
GLIBCXX_3.4.12
GLIBCXX_3.4.2
GLIBCXX_3.4.6
GLIBCXX_3.4.15
GLIBCXX_3.4.19
GLIBCXX_3.4.27
GLIBCXX_3.4.23
GLIBCXX_3.4.3
GLIBCXX_3.4.7
GLIBCXX_3.4.18
GLIBCXX_3.4.22
GLIBCXX_3.4.8
GLIBCXX_3.4.13
GLIBCXX_3.4.17
GLIBCXX_3.4.4
GLIBCXX_3.4.26
```
 Comments: 
 Comment 0: Please let me know if any part of issue #12789 applies to this problem.
 Comment 1: Creating explicit rpath entries in the flags: section fixes this problem:

```
    flags:
      cflags: -Wl,-rpath,/path/to/external/compiler's/libstdc++.so/directory
      cxxflags: -Wl,-rpath,/path/to/external/compiler's/libstdc++.so/directory
      fflags: -Wl,-rpath,/path/to/external/compiler's/libstdc++.so/directory
```
This is hinted at in https://spack.readthedocs.io/en/latest/build_systems/intelpackage.html#integrating-external-compilers
{'source': 'GithubIssues', 'repo_name_with_owner': 'spack/spack', 'issue_id': 506771065, 'repo_stars': 2725.0}