[SystemZ][z/OS] Add XPLink ABI on z/OS (#188501)
This patch introduces full support for the XPLINK calling convention on
z/OS within LLVM. XPLINK is documented in the Language Environment
Vendor Interface ([Chapter
22](https://www.ibm.com/docs/en/zos/3.2.0?topic=applications-call-linkage-convention-amode-64))
and in the IBM Redbook [XPLink: OS/390 Extra Performance
Linkage](http://www.redbooks.ibm.com/abstracts/sg245991.html?Open
). It defines a high‑performance linkage model used by the z/OS Language
Environment (LE) and by the XL C/C++ compiler.
This work succeeds and replaces the earlier effort in [PR
101024](https://github.com/llvm/llvm-project/pull/101024). The present
implementation restructures the original approach, significantly expands
test coverage, and aligns more closely with both the architectural
requirements of XPLINK and the de‑facto ABI behaviour of XL C/C++.
