[AIX][llvm][support] Implement getHostCPUName

We implement getHostCPUName() for AIX via systemcfg interfaces since access to the processor version register is a privileged operation. We return a value based on the  current processor implementation mode.

This fixes the cpu detection used by clang for `-mcpu=native`.

Reviewed By: hubert.reinterpretcast

Differential Revision: https://reviews.llvm.org/D95966