common-trampoline: use register keyword to force register allocation

arch/common/common-trampoline.c

@@ -17,9 +17,8 @@
 void
 libucontext_trampoline(void)
 {
-	libucontext_ucontext_t *uc_link;
+	register libucontext_ucontext_t *uc_link;
 
-    // FIXME: there's no guarantee that input is not clobbered!
 	FETCH_LINKPTR(uc_link);
 
 	if (uc_link == NULL)