[SystemZ][z/OS] Save (and restore) R3 to avoid clobbering parameter when call stack...

[SystemZ][z/OS] Save (and restore) R3 to avoid clobbering parameter when call stack frame extension is invoked

When the stack frame extension routine is used, the contents of r3 is overwritten.
However, if r3 is live in the prologue (ie. one of the function's parameters
resides in r3), it needs to be saved. We save r3 in r0 if r0 is available
(ie. r0 is not used as temporary storage for r4), and in the corresponding
stack slot for the third parameter otherwise.

Differential Revision: https://reviews.llvm.org/D150332

Reviewed By: uweigand