build: Fix rules for non-target std when using snapshot std

Author: brson

mk/target.mk

@@ -173,6 +173,7 @@ $(foreach source,$(CFG_TARGET_TRIPLES),				\
  $(foreach target,$(NON_HOST_TRIPLES),				\
   $(eval $(call TARGET_RT_FROM_WD,0,$(target),$(source)))       \
   $(eval $(call TARGET_CORELIB_FROM_WD,0,$(target),$(source)))  \
+  $(eval $(call TARGET_STDLIB_FROM_WD,0,$(target),$(source)))  \
 ))
 
 # After stage0, always build the stage0 runtime from the working directory