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 -dur duma_2_5_1.orig/Makefile duma_2_5_1/Makefile

 --- duma_2_5_1.orig/Makefile	2007-07-11 17:49:43.000000000 +0200

 +++ duma_2_5_1/Makefile	2007-07-11 17:50:46.000000000 +0200

 @@ -115,9 +115,14 @@

  	dumatest.c tstheap.c dumatestpp.cpp testoperators.cpp \

  	createconf.c

 -OBJECTS = dumapp.o duma.o sem_inc.o print.o

 +OBJECTS = duma.o sem_inc.o print.o

 +SO_OBJECTS = duma_so.o sem_inc_so.o print_so.o

 +

 +ifeq ($(DUMA_CPP),1)

 +OBJECTS += dumapp.o

 +SO_OBJECTS += dumapp_so.o

 +endif

 -SO_OBJECTS = dumapp_so.o duma_so.o sem_inc_so.o print_so.o

  all:	libduma.a $(DUMASO) tstheap dumatest dumatestpp testoperators tstheap_so dumatestpp_so

  	@ echo "Testing DUMA."