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>

 timezone data has been split into the package sys-libs/timezone-data

 diff -durN glibc-2.10.1.orig/Makeconfig glibc-2.10.1/Makeconfig

 --- glibc-2.10.1.orig/Makeconfig	2009-11-13 00:50:13.000000000 +0100

 +++ glibc-2.10.1/Makeconfig	2009-11-13 00:50:41.000000000 +0100

 @@ -919,7 +919,7 @@

  	      stdlib stdio-common libio malloc string wcsmbs time dirent    \

  	      grp pwd posix io termios resource misc socket sysvipc gmon    \

  	      gnulib iconv iconvdata wctype manual shadow gshadow po argp   \

 -	      crypt nss localedata timezone rt conform debug		    \

 +	      crypt nss localedata rt conform debug		    \

  	      $(add-on-subdirs) $(dlfcn) $(binfmt-subdir)

  ifndef avoid-generated