[SystemZ/z/OS] Add XPLINK 64-bit calling convention to tablegen.

This commit adds the initial changes to the SystemZ target
description for the XPLINK 64-bit calling convention on z/OS.
Additions include:

 - a new predicate IsTargetXPLINK64
 - different register allocation order
 - generaton of nopr after a call

Reviewed-by: uweigand

Differential Revision: https://reviews.llvm.org/D96887