[SystemZ] Support builtin_frame_address with packed stack without backchain.

In order to use __builtin_frame_address(0) with packed stack and no
backchain, the address of where the backchain would have been written is
returned (like GCC).

This address may either contain a saved register or be unused.

Review: Ulrich Weigand

Differential Revision: https://reviews.llvm.org/D101897