[llvm] [MC] Update '-mcpu=native' test to be Linux only. (PR #159838)

[Cameron McInally via llvm-commits]

mcinally wrote:

@gkistanova, does this seem reasonable to you? I am not a Windows expert by any means.

I suspect that `llvm::sys::getHostCPUName()` and `llvm::sys::getHostCPUFeatures()` need some work for Windows, but it's outside the scope of the project I'm working on.

https://github.com/llvm/llvm-project/pull/159838