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 strace-4.5.15.orig/signal.c strace-4.5.15/signal.c

 --- strace-4.5.15.orig/signal.c	2007-01-11 23:08:38.000000000 +0100

 +++ strace-4.5.15/signal.c	2007-07-14 19:24:43.000000000 +0200

 @@ -1440,7 +1440,7 @@

  	  	tcp->u_rval = tcp->u_error = 0;

  		if(tcp->u_arg[0] == 0)

  		  	return 0;

 -		tcp->auxstr = sprintsigmask("mask now ", tcp->u_arg[1]);

 +		tcp->auxstr = sprintsigmask("mask now ", tcp->u_arg[1], 0);

  		return RVAL_NONE | RVAL_STR;

  	}

  	return 0;