rustc -> $(RUSTC) in rmake test

Author: Jorge Aparicio

src/test/run-make/target-without-atomics/Makefile

@@ -2,4 +2,4 @@
 
 # The target used below doesn't support atomic operations. Verify that's the case
 all:
-	rustc --print cfg --target thumbv6m-none-eabi | grep -qv target_has_atomic
+	$(RUSTC) --print cfg --target thumbv6m-none-eabi | grep -qv target_has_atomic