[SystemZ] Separate LoZ ELF specifics in tablegen.

Separate the LoZ ELF calling convention in tablegen.
This will make it easier to add the z/OS ABI in future patches.

Reviewed By: uweigand

Differential Revision: https://reviews.llvm.org/D96867

GitOrigin-RevId: 8b624a316491f2be114e0a9507dc4ac7a22d016a
13 files changed