cc/gcc: add the backend/frontend infra for final gcc

Currently, we issue the bare-metal compiler from the pass_1 & pass_2
core compilers, because the final gcc breaks while doing so.

This implies we have to build some libces during the start_files step,
instead of the standard libc step. This is the case for newlib.

By adding a backend/frontend infra to the final gcc, we can abstract
what backend to call: the standard backend for non-bare-metal gcc,
and the core backend for bare-metal.

This patch is just an no-op, it just adds the final backend and
frontend without changing the way bare-metal is built, to come in a
subsequent patch.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 diff -durN gcc-4.4.0.orig/libstdc++-v3/include/c_global/cstdio gcc-4.4.0/libstdc++-v3/include/c_global/cstdio

 --- gcc-4.4.0.orig/libstdc++-v3/include/c_global/cstdio	2009-04-10 01:23:07.000000000 +0200

 +++ gcc-4.4.0/libstdc++-v3/include/c_global/cstdio	2009-05-27 21:38:58.000000000 +0200

 @@ -139,7 +139,7 @@

  _GLIBCXX_END_NAMESPACE

 -#if _GLIBCXX_USE_C99

 +#if _GLIBCXX_USE_C99 || defined __UCLIBC__

  #undef snprintf

  #undef vfscanf