[clang] [AIX] Detect `#pragma mc_func` (PR #99888)

[Chen Zheng via cfe-commits]

chenzheng1030 wrote:

ee07547487a3502f7436968bbfc243b054c14071 should be able to fix the `No available targets are compatible with triple "powerpc64-ibm-aix"` error.

https://github.com/llvm/llvm-project/pull/99888