[z/OS] Implement prologue and epilogue generation for z/OS target.

This patch adds support for prologue and epilogue generation for
the z/OS target under the XPLINK64 ABI for functions with a stack
size of less than 1048576 bytes (huge stack frames).

Reviewed by: uweigand, Kai

Differential Revision: https://reviews.llvm.org/D114457