[clang] [SystemZ][z/OS] Implement z/OS XPLINK ABI (PR #91384)

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic commented:

At first glance, making a separate ABI seemed weird, but I guess it's pretty different from the normal SystemZ ABI.

https://github.com/llvm/llvm-project/pull/91384