Issue title: [grpc]: Always insists on building/installing dependencies for x64-windows even if not using x64-windows triplet
 Issue body: **Describe the bug**

When wanting to `vcpkg install grpc` for any windows triplet that is not `x64-windows`. The installation plan still insists on installing certain dependencies of `grpc` for `x64-windows`

**Environment**
- OS: Windows
- Compiler: MSVC 2019
- Vcpkg: Current `master` (as of: 2021-07-21)

**To Reproduce**
Steps to reproduce the behavior:

1. Install `grpc` with any windows triplet that is not `x64-windows`. eg: `vcpkg.exe install grpc:x64-windows-static`
2. It will produce the following installation plan:

```
Computing installation plan...
The following packages will be built and installed:
  * abseil[core]:x64-windows -> 2021-03-24#1
  * abseil[core]:x64-windows-static -> 2021-03-24#1
  * c-ares[core]:x64-windows -> 1.17.1#1
  * c-ares[core]:x64-windows-static -> 1.17.1#1
  * grpc[codegen,core]:x64-windows -> 1.37.0#2
    grpc[core]:x64-windows-static -> 1.37.0#2
  * openssl[core]:x64-windows -> 1.1.1k#7
  * openssl[core]:x64-windows-static -> 1.1.1k#7
  * protobuf[core]:x64-windows -> 3.15.8#3
  * protobuf[core]:x64-windows-static -> 3.15.8#3
  * re2[core]:x64-windows -> 2020-10-01
  * re2[core]:x64-windows-static -> 2020-10-01
  * upb[core]:x64-windows -> 2020-12-19#1
  * upb[core]:x64-windows-static -> 2020-12-19#1
  * zlib[core]:x64-windows -> 1.2.11#10
  * zlib[core]:x64-windows-static -> 1.2.11#10
Additional packages (*) will be modified to complete this operation.
```

**Expected behavior**

I would've expected the installation plan to list dependencies to be built/installed for `x64-windows-static` or any other triplet (built-in or custom) that I specified.

**Failure logs** 

N/A

**Additional context**

N/A
 Comments: 
 Comment 0: This is due to `grpc` having a host dependency on itself, and the default host triplet is `x64-windows`
Add `--host-triplet=x64-windows-static` to the vcpkg command
 Comment 1: Thanks for the explanation @longnguyen2004  

I guess my main concern is when using `vcpkg export` where libs for the host triplet (`x64-windows`) are also included and I am very conscious about the final size of the vcpkg export archive being bloated up with libs for a triplet I'm not targeting/consuming. Is this intended behaviour as well?


 Comment 2: Seems to be the same as https://github.com/microsoft/vcpkg/issues/17555
 Comment 3: - [ ] 1888
 Comment 4: There are cases where ports require tools from other ports in order to build (sometimes even from themselves). So, we built the tools for the host and reuse them for other ports that need them.

It is unfortunate but right now you can't uninstall the host package without having to remove the target package. But we would gladly accept a PR that changes the tool to allow that.
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