[clang] [FMV][AIX] Implement target_clones (PR #177428)

[Zarko Todorovski via cfe-commits]

ZarkoT wrote:

As discussed offline we also need:
1. Documentation describing how to use the attribute (what's accepted, etc)
2. Tests checking if the Function Attributes of the cloned functions for each target cpu match those of functions just generated with just using `-mcpu=pwrN`

https://github.com/llvm/llvm-project/pull/177428