[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.
----------------
mcinally wrote:

I considered that, but it is a functional change. An unspecified -mcpu gives the baseline CPU, where that change would return the host CPU. It probably needs a whole RFC before changing and I'm not even sure if it's desirable in general. A compiler like GCC won't give you the host CPU by default (which is why `-mcpu=native` exists).

This patch is more in line with how the Frontends deal with `-mcpu=native`. E.g. clang/lib/Driver/ToolChains/Arch/AArch64.cpp:

```
  // Handle CPU name is 'native'.
  if (CPU == "native")
    return std::string(llvm::sys::getHostCPUName());
```

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