runtime: fix typo in ARM code

The typo was introduced by one of Dmitriy's CLs this morning.
The fix makes the ARM build compile again; it still won't pass
its tests, but one thing at a time.

TBR=dvyukov
CC=golang-codereviews
https://golang.org/cl/55770044

src/pkg/runtime/atomic_arm.c

@@ -49,7 +49,7 @@ runtime·xchgp(void* volatile* addr, void* v)
 
 	for(;;) {
 		old = *addr;
-		if(runtime·cas(addr, old, v))
+		if(runtime·casp(addr, old, v))
 			return old;
 	}
 }