Config.mk: Provide the default MODULECC/MODULELD

In some cases, a separate toolchain is necessary to
build modules.
E.g. the sim on macOS, where the native format is Mach-O.

By default, just use CC/LD, which is appropriate for
the most of platforms.

tools/Config.mk

@@ -363,3 +363,9 @@ define TESTANDREPLACEFILE
 	fi
 endef
 endif
+
+# Some defaults.
+# $(TOPDIR)/Make.defs can override these appropriately.
+
+MODULECC = $(CC)
+MODULELD = $(LD)