cc/gcc: do not use the core pass-2 to build the baremetal compiler

In case we build a baremetal compiler, use the standard passes:
- core_cc is used to build the C library;
- as such, it is meant to run on build, not host;
- the final compiler is meant to run on host;

As the current final compiler step can not build a baremetal compiler,
call the core backend from the final step.

NB: Currently, newlib is built during the start_files pass, so we have
to have a core compiler by then... Once we can build the baremetal
compiler from the final cc step, then we can move the newlib build to
the proper step, and then get rid of the core pass-1 static compiler...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # Menu for the native gdbserver

 config GDB_GDBSERVER

     bool

     prompt "gdbserver"

     depends on ! BARE_METAL

     help

       Build and install a gdbserver for the target, to run on the target.

 if GDB_GDBSERVER

 config GDB_GDBSERVER_STATIC

     bool

     prompt "Build a static gdbserver"

     default y

     help

       In case you have trouble with dynamic loading of shared libraries,

       you will find that a static gdbserver comes in handy.

 endif # GDB_GDBSERVER

 if BARE_METAL

 comment "In bare-metal, you'll need to   "

 comment "provide your own gdbserver stub."

 endif # BARE_METAL