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 H.J. Lu, as reported there:

 http://www.nabble.com/PATCH:-Use-short-for-fnstsw-td14775171.html

 fnstsw stores 16bit into %ax. The upper 16bit of %eax is unchanged.

 The new assembler (binutils-2.18.50.0.4 and up) will disallow "fnstsw %eax".

 diff -dur glibc-2.6.1.orig/sysdeps/i386/fpu/ftestexcept.c glibc-2.6.1/sysdeps/i386/fpu/ftestexcept.c

 --- glibc-2.6.1.orig/sysdeps/i386/fpu/ftestexcept.c	2004-03-05 11:14:48.000000000 +0100

 +++ glibc-2.6.1/sysdeps/i386/fpu/ftestexcept.c	2008-06-24 10:56:14.000000000 +0200

 @@ -26,7 +26,7 @@

  int

  fetestexcept (int excepts)

  {

 -  int temp;

 +  short temp;

    int xtemp = 0;

    /* Get current exceptions.  */