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>

 # libelf config file

 choice

     bool

     prompt "libelf version"

 # Don't remove next line

 # CT_INSERT_VERSION_BELOW

 config LIBELF_V_0_8_13

     bool

     prompt "0.8.13"

 config LIBELF_V_0_8_12

     bool

     prompt "0.8.12"

 endchoice

 config LIBELF_VERSION

     string

 # Don't remove next line

 # CT_INSERT_VERSION_STRING_BELOW

     default "0.8.13" if LIBELF_V_0_8_13

     default "0.8.12" if LIBELF_V_0_8_12