cmd/compile: fix no-op instruction used by s390x

CL 152537 introduced a new use for ginsnop, the arch-dependent
routine that generates nops. The previous s390x nop clobbered flags.
It turns out the previous uses of this nop did not require flags
to be preserved, but the new use does.

Use a real nop: the 4-byte preferred nop.

Fixes #29453

Change-Id: I95310dfdd831932e26f5d5b6608324687f4c3162
Reviewed-on: https://go-review.googlesource.com/c/155926


Reviewed-by: Michael Munday <mike.munday@ibm.com>