build libcore with --enforce-mut-vars

Author: nikomatsakis

mk/target.mk

@@ -31,7 +31,7 @@ $$(TLIB$(1)_T_$(2)_H_$(3))/$$(CFG_CORELIB): \
 		$$(CORELIB_CRATE) $$(CORELIB_INPUTS) \
 		$$(TSREQ$(1)_T_$(2)_H_$(3))
 	@$$(call E, compile_and_link: $$@)
-	$$(STAGE$(1)_T_$(2)_H_$(3)) -o $$@ $$< && touch $$@
+	$$(STAGE$(1)_T_$(2)_H_$(3)) --enforce-mut-vars -o $$@ $$< && touch $$@
 
 $$(TLIB$(1)_T_$(2)_H_$(3))/$$(CFG_STDLIB): \
 		$$(STDLIB_CRATE) $$(STDLIB_INPUTS) \