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>