Help ensure that the copy loop doesn't get converted to a memcpy() call.

Reported and reviewed by: kib
X-MFC with:	r337715
Sponsored by:	The FreeBSD Foundation

sys/x86/x86/ucode.c

@@ -346,7 +346,7 @@ ucode_load_bsp(uintptr_t free)
 			addr = map_ucode(free, len);
 			/* We can't use memcpy() before ifunc resolution. */
 			for (i = 0; i < len; i++)
-				addr[i] = match[i];
+				addr[i] = ((volatile uint8_t *)match)[i];
 			match = addr;
 
 			if (loader->load(match, false) == 0) {