runtime: Plan 9: fix errstr

The call to the C function runtime.findnull() requires
that we provide the argument at 0(SP).

R=rsc, rminnich, ality
CC=golang-dev
https://golang.org/cl/7559047

src/pkg/runtime/sys_plan9_386.s

@@ -187,6 +187,13 @@ TEXT runtime·errstr(SB),7,$0
 	MOVL	$ERRMAX, 8(SP)
 	MOVL	$41, AX
 	INT	$64
+
+	// syscall requires caller-save
+	MOVL	4(SP), CX
+
+	// push the argument
+	PUSHL	CX
 	CALL	runtime·findnull(SB)
+	POPL	CX
 	MOVL	AX, 8(SP)
 	RET

src/pkg/runtime/sys_plan9_amd64.s

@@ -224,6 +224,13 @@ TEXT runtime·errstr(SB),7,$0
 	MOVQ	$0x8000, AX
 	MOVQ	$41, BP
 	SYSCALL
+
+	// syscall requires caller-save
+	MOVQ	8(SP), CX
+
+	// push the argument
+	PUSHQ	CX
 	CALL	runtime·findnull(SB)
+	POPQ	CX
 	MOVQ	AX, 16(SP)
 	RET