title: unknown feature `llvm_asm` when compile rust-src
body: <p>I tried to compile rust-src using cargo xbuild but get this error:</p>
  
  <pre class="lang-none prettyprint-override"><code>error[E0635]: unknown feature `llvm_asm`
  -&gt; .cargo/registry/src/github.com-1ecc6299db9ec823/compiler_builtins-0.1.28/src/lib.rs:3:12
  3 | #![feature(llvm_asm)]
  </code></pre>
  
  <p>How can I fix this error? It's seem like xbuild tries to compile the new rust-src with an old rustc. I want it to also use the old rust-src.</p>
  
  <p>I can't update to a newer rustc version as it results in lots of "R_x86_32 relocation" errors, so I would prefer to use the 2020-03-24 version.</p>
  
  <h3>Minimal example</h3>
  
  <p><strong>command</strong></p>
  
  <pre class="lang-none prettyprint-override"><code>cargo new --bin test
  
  rustup component add rust-src
  
  cargo install cargo-xbuild
  
  cd test
  
  ls test
  Cargo.toml  rust-toolchain  src  x86_64-unknown-none.json
  </code></pre>
  
  <p><strong>rust-toolchain</strong></p>
  
  <pre><code>nightly-2020-03-24
  </code></pre>
  
  <p><strong>x86_64-unknown-none.json</strong></p>
  
  <pre><code>{
    "llvm-target": "x86_64-unknown-none",
    "data-layout": "e-m:e-i64:64-f80:128-n8:16:32:64-S128",
    "arch": "x86_64",
    "target-endian": "little",
    "target-pointer-width": "64",
    "target-c-int-width": "32",
    "os": "none",
    "executables": true,
    "linker-flavor": "ld.lld",
    "linker": "rust-lld",
    "panic-strategy": "abort",
    "disable-redzone": true,
    "features": "-mmx,-sse,+soft-float"
  }
  </code></pre>
  
  <p><strong>src/main.rs</strong></p>
  
  <pre><code>#![no_std]                // don't link the Rust standard library
  #![no_main]               // disable all Rust-level entry points
  #![allow(non_snake_case)] // disable non snake case name warning
  
  use core::panic::PanicInfo;
  
  #[no_mangle]
  pub extern "C" fn _start() -&gt; ! {
      loop {}
  }
  
  #[panic_handler]
  pub fn MyPacnicHandler(_panicInfo: &amp;PanicInfo) -&gt; ! {
      loop {}
  }
  </code></pre>
  
  <p><strong>compile</strong></p>
  
  <pre class="lang-none prettyprint-override"><code>cargo xbuild --target x86_64-unknown-none
  </code></pre>
  
  <p><strong>rustc --version</strong></p>
  
  <pre class="lang-none prettyprint-override"><code>rustc 1.44.0-nightly (1edd389cc 2020-03-23)
  </code></pre>