[toolchain] invoke patch-specs.sh in the gcc/final install stage.

Patch up OpenWrts own cross toolchains to always search headers
and libraries in $STAGING_DIR, this should solve most issues with
missing headers, indirect linking and not found libraries.
At a later stage, all -I and -L flags will be purged from
TARGET_LDFLAGS and TARGET_CPPFLAGS.

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@29768 3c298f89-4303-0410-b956-a3cf2f4a3e73