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 Gentoo.

 -= BEGIN original header =-

 -= END original header =-

 diff -durN uClibc-0.9.30.orig/Rules.mak uClibc-0.9.30/Rules.mak

 --- uClibc-0.9.30.orig/Rules.mak	2008-11-12 13:24:16.000000000 +0100

 +++ uClibc-0.9.30/Rules.mak	2009-02-07 09:57:59.000000000 +0100

 @@ -502,6 +502,7 @@

  endif

  CFLAGS += $(call check_gcc,-std=gnu99,)

 +CFLAGS += $(call check_gcc,-fgnu89-inline,)

  LDFLAGS_NOSTRIP:=$(CPU_LDFLAGS-y) -Wl,-shared \

  	-Wl,--warn-common -Wl,--warn-once -Wl,-z,combreloc