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 -dur binutils-2.17.old/bfd/opncls.c binutils-2.17/bfd/opncls.c

--- binutils-2.17.old/bfd/opncls.c	2006-03-16 13:20:16.000000000 +0100

+++ binutils-2.17/bfd/opncls.c	2007-05-01 18:26:11.000000000 +0200

@@ -158,6 +158,13 @@

 {

   bfd *nbfd;

   const bfd_target *target_vec;

+  struct stat s;

+

+  if (stat (filename, &s) == 0)

+    if (S_ISDIR(s.st_mode)) {

+      bfd_set_error (bfd_error_file_not_recognized);

+      return NULL;

+    }

   nbfd = _bfd_new_bfd ();

   if (nbfd == NULL)