Add writeback code to each return location which copies
the final result back to the correct stack location.

Cgo plays tricky games by taking the address of a
in f(a int) (b int) and then using that address to
modify b.  So for cgo-generated Go code, disable the
SSAing of output args.

Update #14511

Change-Id: I95cba727d53699d31124eef41db0e03935862be9
Reviewed-on: https://go-review.googlesource.com/19988Reviewed-by: Todd Neal <todd@tneal.org>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>