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

 CT_EXPERIMENTAL=y

 CT_LOCAL_TARBALLS_DIR="${HOME}/src"

 CT_SAVE_TARBALLS=y

 CT_LOG_EXTRA=y

 CT_ARCH_m68k=y

 CT_KERNEL_linux=y

 CT_KERNEL_V_3_5=y

 CT_BINUTILS_V_2_21_1a=y

 CT_LIBC_UCLIBC_CONFIG_FILE="${CT_LIB_DIR}/samples/${CT_TARGET}/${CT_LIBC}-${CT_LIBC_VERSION}.config"

 CT_THREADS_LINUXTHREADS=y