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

# Menu for the native GDB

config GDB_NATIVE

    bool

    prompt "Native gdb"

    depends on ! BARE_METAL

    depends on ! BACKEND

    help

      Build and install a native gdb for the target, to run on the target.

if GDB_NATIVE

config GDB_NATIVE_STATIC

    bool

    prompt "Build a static native gdb"

    help

      In case you have trouble with dynamic loading of shared libraries,

      you will find that a static gdb comes in handy.

endif # GDB_NATIVE