[dev.power64] runtime: test CAS on large unsigned 32-bit numbers

This adds a test to runtime·check to ensure CAS of large
unsigned 32-bit numbers does not accidentally sign-extend its
arguments.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/162490044

src/runtime/runtime.c

@@ -226,6 +226,12 @@ runtime·check(void)
 	if(z != 4)
 		runtime·throw("cas4");
 
+	z = 0xffffffff;
+	if(!runtime·cas(&z, 0xffffffff, 0xfffffffe))
+		runtime·throw("cas5");
+	if(z != 0xfffffffe)
+		runtime·throw("cas6");
+
 	k = (byte*)0xfedcb123;
 	if(sizeof(void*) == 8)
 		k = (byte*)((uintptr)k<<10);