[SystemZ][z/OS] Implement getHostCPUName for z/OS

- Currently, the host cpu information is not easily available on z/OS as in other platforms.
- This information is stored in the Communications Vector Table (https://www.ibm.com/docs/en/zos/2.2.0?topic=information-cvt-mapping)

Reviewed By: uweigand

Differential Revision: https://reviews.llvm.org/D102793