[SystemZ] [z/OS] Add SystemZCallingConventionRegisters class

This patch adds the abstract class SystemZCallingConventionRegisters
which is a SystemZ-specific class detailing special registers used
by calling conventions on the target. SystemZELFRegisters and
SystemZXPLINK64Registers implement this class for ELF and XPLINK64
respectively.

Reviewed By: uweigand

Differential Revision: https://reviews.llvm.org/D102370