[dev.power64] runtime: fix early GC of Defer objects

go_bootstrap was panicking during runtime initialization
(under runtime.main) because Defer objects were being
prematurely GC'd.  This happened because of an incorrect
change to runtime·unrollgcprog_m to make it endian-agnostic
during the conversion of runtime bitmaps to byte arrays.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/161960044

src/runtime/mgc0.c

@@ -1799,7 +1799,7 @@ runtime·unrollgcprog_m(void)
 		}
 		
 		// atomic way to say mask[0] = 1
-		x = typ->gc[0];
+		x = *(uintptr*)mask;
 		((byte*)&x)[0] = 1;
 		runtime·atomicstorep((void**)mask, (void*)x);
 	}