6648 illumos build should be explicit about C standards

@@ -1298,11 +1298,11 @@
 assym.h := CFLAGS64 += -g
 
 GENASSYM_C = $(LIBCDIR)/$(MACH)/genassym.c
 
 genassym: $(GENASSYM_C)
-        $(NATIVECC) -I$(LIBCBASE)/inc -I$(LIBCDIR)/inc \
+        $(NATIVECC) $(NATIVE_CFLAGS) -I$(LIBCBASE)/inc -I$(LIBCDIR)/inc \
                 $(CPPFLAGS.native) -o $@ $(GENASSYM_C)
 
 OFFSETS = $(LIBCDIR)/$(MACH)/offsets.in
 
 assym.h: $(OFFSETS) genassym