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>

 # mingw32 config options

 ## depends on EXPERIMENTAL && ( ARCH_x86 && ARCH_32 )

 ##

 ## select MINGW32

 ##

 ## help Build a toolchain targeting systems running Windows as host

 choice

     bool

     prompt "Windows api version"

 # Don't remove next line

 # CT_INSERT_VERSION_BELOW

 config W32API_V_3_14

     bool

     prompt "3.14"

 config W32API_V_select

     bool

     prompt "Other version"

 endchoice

 config W32API_VERSION

     string

     prompt "W32 api version" if W32API_V_select

 # Don't remove next line

 # CT_INSERT_VERSION_STRING_BELOW

     default "3.14" if W32API_V_3_14

     help

       Enter the version number of the windows api files to use