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 http://www.nabble.com/arm-linux-compilation-failure-and-possible-fix-td19229074.html

 diff -durN glibc-2.9.orig/ports/sysdeps/unix/sysv/linux/arm/sysdep.h glibc-2.9/ports/sysdeps/unix/sysv/linux/arm/sysdep.h

 --- glibc-2.9.orig/ports/sysdeps/unix/sysv/linux/arm/sysdep.h 25 Nov 2008 16:37:26 -0000 1.33

 +++ glibc-2.9/ports/sysdeps/unix/sysv/linux/arm/sysdep.h 27 Jan 2009 15:35:38 -0000

 @@ -28,6 +28,8 @@

  /* Defines RTLD_PRIVATE_ERRNO and USE_DL_SYSINFO.  */

  #include <dl-sysdep.h>

 +#include <tls.h>

 +

  /* For Linux we can use the system call table in the header file

 	/usr/include/asm/unistd.h

     of the kernel.  But these symbols do not follow the SYS_* syntax