runtime: fix arm build

The current failure is:
fatal error: runtime: stack split during syscall
goroutine 2 [stack split]:
_addv(0xb6fa0f28, 0xd0a5112e, 0x13156d6e, 0xf8475800, 0xd)
        /usr/local/go/src/pkg/runtime/vlrt_arm.c:66 fp=0xb6fa0ef8
notetsleep(0xb6fa0f9c, 0xf8475800, 0xd, 0x0, 0x0, ...)
        /usr/local/go/src/pkg/runtime/lock_futex.c:156 +0xd0 fp=0xb6fa0f18
runtime.notetsleepg(0xb6fa0f9c, 0xf8475800, 0xd)
        /usr/local/go/src/pkg/runtime/lock_futex.c:197 +0x74 fp=0xb6fa0f3c

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/12052043

src/pkg/runtime/vlop_arm.s

@@ -27,7 +27,7 @@ arg=0
 
 /* replaced use of R10 by R11 because the former can be the data segment base register */
 
-TEXT _mulv(SB), $0
+TEXT _mulv(SB), 7, $0
 	MOVW	0(FP), R0
 	MOVW	4(FP), R2	/* l0 */
 	MOVW	8(FP), R11	/* h0 */

src/pkg/runtime/vlrt_arm.c

@@ -62,6 +62,7 @@ struct  Vlong
 
 void    runtime·abort(void);
 
+#pragma textflag 7
 void
 _addv(Vlong *r, Vlong a, Vlong b)
 {