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>

 # Test suite config options

 if EXPERIMENTAL

 menu "Test suite"

 config TEST_SUITE

     bool

 config TEST_SUITE_GCC

     bool

     prompt "GCC test suite"

     depends on EXPERIMENTAL

     select TEST_SUITE

     help

       Select this option to install the GCC test suite in $CT_PREFIX_DIR/test_suite.

       The GCC test suite includes a collection of various toolchain tests for GCC -

       it utilizes the DejaGnu test framework.

       For some tests a network enabled target with ssh server is required.

       A helper Makefile is provided for running the tests - please see the included

       README for information on how to run the test suite.

 endmenu

 endif