[llvm] [MC] Add support for -mcpu=native. (PR #159414)

[Cameron McInally via llvm-commits]

================
@@ -467,6 +467,10 @@ int main(int argc, char **argv) {
     FeaturesStr = Features.getString();
   }
 
+  // Replace -mcpu=native with Host CPU.
+  if (MCPU == "native")
+    MCPU = std::string(llvm::sys::getHostCPUName());
+
----------------
mcinally wrote:

> Clang does use getHostCPUFeatures with -mcpu=native. I wrote the code originally. It's here.

That's `-march` now. Maybe changed when `-mcpu` was deprecated on X86.

```
$ clang test.c -mcpu=native
clang: error: unsupported option '-mcpu=' for target 'x86_64-unknown-linux-gnu'
```

https://github.com/llvm/llvm-project/pull/159414