runtime: fix s390x asm frame sizes

Found by vet.

Change-Id: I1d78454facdd3522509ecfe7c73b21c4602ced8a
Reviewed-on: https://go-review.googlesource.com/32670
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Munday <munday@ca.ibm.com>

src/runtime/asm_s390x.s

@@ -1008,7 +1008,7 @@ TEXT runtime·prefetcht2(SB),NOSPLIT,$0-8
 TEXT runtime·prefetchnta(SB),NOSPLIT,$0-8
 	RET
 
-TEXT runtime·sigreturn(SB),NOSPLIT,$0-8
+TEXT runtime·sigreturn(SB),NOSPLIT,$0-0
 	RET
 
 TEXT ·publicationBarrier(SB),NOSPLIT|NOFRAME,$0-0

src/runtime/internal/atomic/asm_s390x.s

@@ -69,7 +69,7 @@ TEXT ·Xadduintptr(SB), NOSPLIT, $0-24
 	BR	·Xadd64(SB)
 
 // func Xaddint64(ptr *int64, delta int64) int64
-TEXT ·Xaddint64(SB), NOSPLIT, $0-16
+TEXT ·Xaddint64(SB), NOSPLIT, $0-24
 	BR	·Xadd64(SB)
 
 // func Casp1(ptr *unsafe.Pointer, old, new unsafe.Pointer) bool