[SystemZ] [NFC] Add SystemZELFFrameLowering and SystemZXPLINKFrameLowering classes.

This patch adds class SystemZFrameLowering which is a SystemZ-specific class
detailing special registers used by calling conventions on the target.
SystemZELFFrameLowering and SystemZXPLINKFrameLowering implement this class
for ELF and XPLINK64 respectively. Previous functionality in SystemZFrameLowering
is moved to SystemZELFFrameLowering. SystemZXPLINKFrameLowering can then be
implemented in future patches.

Reviewed By: uweigand, Kai

Differential Revision: https://reviews.llvm.org/D108777