cc/gcc: modify to build gcc-4.8-based cross-tools

Building cross-tool based on gcc-4.8 fails while "Installing
pass-2 core C compiler", because building libgcc.mvars needs
libbacktrace.a that gcc.sh doesn't build. This patch inserts
a few lines configuring, and making libbacktrace into gcc.sh
to build gcc-4.8-based cross-tools successfully.

Reported-by: Plotnikov Dmitry <leitz@ispras.ru>
Signed-off-by: Jongsung Kim <neidhard.kim@lge.com>
Message-Id: <201305031831.33395.neidhard.kim@lge.com>
Patchwork-Id: 241258

 reporter_name="Yann E. MORIN"

 reporter_url="http://ymorin.is-a-geek.org/"

 reporter_comment=""