fix regression that introduced race condition build failures for gcc

commit 20ce424 introduced a situation
where two instances of gcc's build process could be invoked
concurrently in the process of fixing the same issue for musl. only
the production of native compilers seems to have been affected. add a
direct dependency edge rather than relying on the indirect dependency,
which was only present in non-native mode.

litecross/Makefile

@@ -177,7 +177,7 @@ obj_sysroot/.lc_libs: | obj_musl/.lc_built
 	cd obj_musl && $(MAKE) $(MUSL_VARS) DESTDIR=$(PWD)/obj_sysroot install
 	touch $@
 
-obj_toolchain/.lc_built: | obj_toolchain/.lc_configured
+obj_toolchain/.lc_built: | obj_toolchain/.lc_configured obj_toolchain/gcc/.lc_built
 	cd obj_toolchain && $(MAKE) MAKE="$(MAKE)" LC_ROOT=$(PWD)
 	touch $@