LLVM warnings on mips* when using `-C target-cpu=native`

[infinity0]

This is causing the cargo 0.35.0 tests to fail on mips* - e.g. mips64el.

$ rustc --version
rustc 1.34.2
$ cat hello.rs
fn main() {}
$ rustc -C target-cpu=native hello.rs
'generic' is not a recognized processor for this target (ignoring processor)
'generic' is not a recognized processor for this target (ignoring processor)
'generic' is not a recognized processor for this target (ignoring processor)
'generic' is not a recognized processor for this target (ignoring processor)
'generic' is not a recognized processor for this target (ignoring processor)
[..]

[infinity0]

Seems related to #53322 but I wasn't sure how to adapt the fix from there to this.

[infinity0]

@alexcrichton Looks like your solution in #53642 relies on using LLVM's llvm::sys::getHostCPUName however on mips* this returns "generic":

$ cat lol.cc
#include <llvm/Support/Host.h>
#include <stdio.h>

using namespace llvm;

int main() {
  printf("%s\n", sys::getHostCPUName());
}
$ g++ -I/usr/lib/llvm-7/include lol.cc -lLLVM-7 && ./a.out
generic

(on my own intel box this returns skylake)

[workingjubilee]

Checking reports for newer versions like Debian Bullseye, the current stable release, buildd appears to pass this for remaining mips* architectures. Closing on the assumption that this was cleared.