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 glibc-2.10.1.orig/sysdeps/sh/sh4/fpu/fpu_control.h glibc-2.10.1/sysdeps/sh/sh4/fpu/fpu_control.h

 --- glibc-2.10.1.orig/sysdeps/sh/sh4/fpu/fpu_control.h	2001-07-06 06:56:04.000000000 +0200

 +++ glibc-2.10.1/sysdeps/sh/sh4/fpu/fpu_control.h	2009-11-13 00:51:26.000000000 +0100

 @@ -45,6 +45,7 @@

  #define _FPU_GETCW(cw) __asm__ ("sts fpscr,%0" : "=r" (cw))

  #if defined __GNUC__

 +extern void __set_fpscr(unsigned long);

  #define _FPU_SETCW(cw) __set_fpscr ((cw))

  #else

  #define _FPU_SETCW(cw) __asm__ ("lds %0,fpscr" : : "r" (cw))