gcc: Stash the gcc-cross builddir to reuse in libgcc and gcc-runtime
Currently we stash the libgcc install tree and then reuse that
to populate libgcc recipe later. This mechanism does not work
for gcc 4.7/trunk since now libstdc++ needs access to build tree
of libgcc. This patch stashes the gcc-cross build tree
and then reuses this in libgcc as well as in gcc-runtime
recipe builds.
Now we build libgcc in the libgcc recipe instead of just
using the prebuilt install tree
core-image-minimal build/run tested on all qemu machines
