runtime: fix arm build

The current failure is:
fatal error: runtime: stack split during syscall
goroutine 2 [stack split]:
_si2v(0xb6ebaebc, 0x3b9aca00)
        /usr/local/go/src/pkg/runtime/vlrt_arm.c:628 fp=0xb6ebae9c
runtime.timediv(0xf8475800, 0xd, 0x3b9aca00, 0xb6ebaef4)
        /usr/local/go/src/pkg/runtime/runtime.c:424 +0x1c fp=0xb6ebaed4

Just adding textflag 7 causes the following error:
notetsleep: nosplit stack overflow
        128	assumed on entry to notetsleep
        96	after notetsleep uses 32
        60	after runtime.futexsleep uses 36
        4	after runtime.timediv uses 56
        -4	after _si2v uses 8

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/12001045

src/pkg/runtime/vlrt_arm.c

@@ -624,14 +624,12 @@ _ul2v(Vlong *ret, ulong ul)
 	ret->hi = 0;
 }
 
+#pragma textflag 7
 void
 _si2v(Vlong *ret, int si)
 {
-	long t;
-
-	t = si;
-	ret->lo = t;
-	ret->hi = t >> 31;
+	ret->lo = (long)si;
+	ret->hi = (long)si >> 31;
 }
 
 void