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>

src/cmd/compile/internal/ssa/gen/S390XOps.go

@@ -630,6 +630,7 @@ func init() {
 			asm:            "STMG",
 			faultOnNilArg0: true,
 			symEffect:      "Write",
+			clobberFlags:   true, // TODO(mundaym): currently uses AGFI to handle large offsets
 		},
 		{
 			name:           "STMG3",
@@ -640,6 +641,7 @@ func init() {
 			asm:            "STMG",
 			faultOnNilArg0: true,
 			symEffect:      "Write",
+			clobberFlags:   true, // TODO(mundaym): currently uses AGFI to handle large offsets
 		},
 		{
 			name:      "STMG4",
@@ -657,6 +659,7 @@ func init() {
 			asm:            "STMG",
 			faultOnNilArg0: true,
 			symEffect:      "Write",
+			clobberFlags:   true, // TODO(mundaym): currently uses AGFI to handle large offsets
 		},
 		{
 			name:           "STM2",
@@ -667,6 +670,7 @@ func init() {
 			asm:            "STMY",
 			faultOnNilArg0: true,
 			symEffect:      "Write",
+			clobberFlags:   true, // TODO(mundaym): currently uses AGFI to handle large offsets
 		},
 		{
 			name:           "STM3",
@@ -677,6 +681,7 @@ func init() {
 			asm:            "STMY",
 			faultOnNilArg0: true,
 			symEffect:      "Write",
+			clobberFlags:   true, // TODO(mundaym): currently uses AGFI to handle large offsets
 		},
 		{
 			name:      "STM4",
@@ -694,6 +699,7 @@ func init() {
 			asm:            "STMY",
 			faultOnNilArg0: true,
 			symEffect:      "Write",
+			clobberFlags:   true, // TODO(mundaym): currently uses AGFI to handle large offsets
 		},
 
 		// large move