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>

 Original patch from: ../4.3.2/140-netbsd-symbolic.patch

 -= BEGIN original header =-

 Original patch from gentoo: gentoo/src/patchsets/gcc/4.3.1/gentoo/11_all_gcc-netbsd-symbolic.patch

 http://bugs.gentoo.org/122698

 -= END original header =-

 diff -durN gcc-4.3.3.orig/gcc/config/netbsd-elf.h gcc-4.3.3/gcc/config/netbsd-elf.h

 --- gcc-4.3.3.orig/gcc/config/netbsd-elf.h	2007-09-03 18:14:04.000000000 +0200

 +++ gcc-4.3.3/gcc/config/netbsd-elf.h	2009-01-27 22:19:18.000000000 +0100

 @@ -82,6 +82,7 @@

  #define NETBSD_LINK_SPEC_ELF \

    "%{assert*} %{R*} %{rpath*} \

     %{shared:-shared} \

 +   %{symbolic:-Bsymbolic} \

     %{!shared: \

       -dc -dp \

       %{!nostdlib: \