reflect: fix s390x reflect method calls

R0 isn't the zero register any more. Oops.

Update #27695.

Change-Id: I46a975ed37d5e570afe2e228d3edf74949e08ad7
Reviewed-on: https://go-review.googlesource.com/138580Reviewed-by: Michael Munday <mike.munday@ibm.com>

src/reflect/asm_s390x.s

@@ -14,7 +14,7 @@ TEXT ·makeFuncStub(SB),(NOSPLIT|WRAPPER),$32
 	MOVD	R12, 8(R15)
 	MOVD	$argframe+0(FP), R3
 	MOVD	R3, 16(R15)
-	MOVB	R0, 32(R15)
+	MOVB	$0, 32(R15)
 	ADD	$32, R15, R3
 	MOVD	R3, 24(R15)
 	BL	·callReflect(SB)
@@ -29,7 +29,7 @@ TEXT ·methodValueCall(SB),(NOSPLIT|WRAPPER),$32
 	MOVD	R12, 8(R15)
 	MOVD	$argframe+0(FP), R3
 	MOVD	R3, 16(R15)
-	MOVB	R0, 32(R15)
+	MOVB	$0, 32(R15)
 	ADD	$32, R15, R3
 	MOVD	R3, 24(R15)
 	BL	·callMethod(SB)