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-4.5.16.orig/system.c strace-4.5.16/system.c

--- strace-4.5.16.orig/system.c	2007-01-17 00:22:36.000000000 +0100

+++ strace-4.5.16/system.c	2007-07-14 19:28:41.000000000 +0200

@@ -1612,7 +1612,9 @@

 	{ CTL_KERN, "CTL_KERN" },

 	{ CTL_VM, "CTL_VM" },

 	{ CTL_NET, "CTL_NET" },

+#ifdef CTL_PROC

 	{ CTL_PROC, "CTL_PROC" },

+#endif

 	{ CTL_FS, "CTL_FS" },

 	{ CTL_DEBUG, "CTL_DEBUG" },

 	{ CTL_DEV, "CTL_DEV" },