correct ASM_COMMENTS Makefile option

Author: nikomatsakis

Makefile.in

@@ -111,7 +111,7 @@ ifdef SAVE_TEMPS
   CFG_RUSTC_FLAGS += --save-temps
 endif
 ifdef ASM_COMMENTS
-  CFG_RUSTC_FLAGS += -z asm-comments
+  CFG_RUSTC_FLAGS += -Z asm-comments
 endif
 ifdef TIME_PASSES
   CFG_RUSTC_FLAGS += -Z time-passes