target gcc: fix recompilation issue

gcc_target wants uclibc_target which is now first stage uclibc (not
real, from the NPTL build stage addition).
This triggers gcc-final to be rebuilt (albeit cached) and reinstalled,
thus touching all the stamps around and triggering a rebuild of
gcc_target.
Switch to $(STAMP_DIR)/gcc_libs_target_installed instead so that we only
depend on the last stage of cross gcc being installed to rebuild.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

toolchain/gcc/gcc-uclibc-4.x.mk

@@ -558,7 +558,7 @@ $(TARGET_DIR)/usr/bin/gcc: $(GCC_BUILD_DIR4)/.compiled
 	#rm -rf $(TARGET_DIR)/usr/lib/libgcc_s*.so*
 	touch -c $@
 
-gcc_target: uclibc_target $(GCC_TARGET_PREREQ) binutils $(TARGET_DIR)/usr/bin/gcc
+gcc_target: $(STAMP_DIR)/gcc_libs_target_installed $(GCC_TARGET_PREREQ) binutils $(TARGET_DIR)/usr/bin/gcc
 
 gcc_target-clean:
 	rm -rf $(GCC_BUILD_DIR4)