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>

 # MIPS specific config options

 choice

     bool

     prompt "ABI"

 config ARCH_mips_o32

     bool

     prompt "o32"

     depends on ARCH_32

     help

       This is the -mabi=32 gcc option.

 config ARCH_mips_n32

     bool

     prompt "n32"

     depends on ARCH_64

     help

       This is the -mabi=n32 gcc option.

 config ARCH_mips_n64

     bool

     prompt "n64"

     depends on ARCH_64

     help

       This is the -mabi=64 gcc option.

 # Not supported on Linux:

 # o64   : seems related to *BSD

 # eabi  : seems related to bare-metal

 endchoice

 config ARCH_mips_ABI

     string

     default "32"    if ARCH_mips_o32

     default "n32"   if ARCH_mips_n32

     default "64"    if ARCH_mips_n64