[SystemZ/z/OS] Add alias for XPLINK return

The XPLINK return `b 2(7)` has size 4 bytes, while the Linux return
`br 7` only has size 2 bytes. Thus a new alias is required to have correct
instruction byte count. It also fixes the conditional return code.

Reviewed By: uweigand

Differential Revision: https://reviews.llvm.org/D119437