cmd/compile: mark 'store multiple' as clobbering flags on s390x

Store multiple instructions can clobber flags on s390x when the
offset passed into the assembler is outside the range representable
with a signed 20 bit integer. This is because the assembler uses
the agfi instruction to implement the large offset. The assembler
could use a different sequence of instructions, but for now just
mark the instruction as 'clobberFlags' since this is risk free.

Noticed while investigating #38195.

No test yet since I'm not sure how to get this bug to trigger and
I haven't seen it affect real code.

Change-Id: I4a6ab96455a3ef8ffacb76ef0166b97eb40ff925
Reviewed-on: https://go-review.googlesource.com/c/go/+/226759


Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>