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

[Cameron McInally via llvm-commits]

================
@@ -0,0 +1,3 @@
+# RUN: llvm-mc -filetype=obj -mcpu=native %s 2>&1 | FileCheck %s
+
+# CHECK-NOT: 'native' is not a recognized processor for this target (ignoring processor)
----------------
mcinally wrote:

Looks like FileCheck doesn't like an empty input file (really stderr piped to stdout):

```
FileCheck error: '<stdin>' is empty.
```

I could make the CHECK-NOT more narrow. Maybe something like:

```
# CHECK-NOT: native
```

Does that seem acceptable, @arsenm?

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