[clang] [llvm] [AArch64][Driver] Fix behavior of +nofeat modifier on -mcpu (PR #203458)

[via cfe-commits]

yamash-fj wrote:

Thanks for the review.

This change is intended to affect -mcpu only; it does not change the
behavior of ``-march``. I will update the PR description to make this explicit.

I checked the existing test coverage for ``-march`` ``+nofeat`` behavior and found some existing
tests, for example in ``clang/test/Driver/arm-sb.c``. 

https://github.com/llvm/llvm-project/pull/203458