[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