cc/gcc: baremetal requires a two-pass process

Here, we implement a highly ugly hack. I'm not proud of that one...

To build the libstdc++ library, the compiler requires the C library. In
case we build for non-baremetal, this is normally handled by the final
step, later.

But in the case of bare-metal, we never go through the final step (because
it does not work, and it seems complex enough to make it work), so the
baremetal compilers are issued out of the core step.

# Dummy config file to not use a C library *at all*

# depends on BARE_METAL

config LIBC_none

    help

      Do not use a C library.

      This is usefull if your project is self-contained, does not rely on

      an external C library, and provides all the necessary bits.

      Most probably usefull to bootloaders, as they generally don't depend

      on the C library.

      If unsure: do *not* choose that, and use another option in the choice.