cc/gcc: build core compilers for canadian

Currently, we rely on an existing external cross-compiler targetting
the target, to build the C library.

This can pause quite a few problems if that compiler is different from
the one we are building, because it could introduce some ABI issues.

This patch removes this dependency, by building the core compilers
as we do for standard cross, and also by building the binutils and
gcc, for running on the build machine.

This means we no longer need to offer the cross-sompiler selection in
the menuconfig.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

# Options specific to crosstool-NG acting as a backend

config IS_A_BACKEND

    string

    option env="CT_IS_A_BACKEND"

config BACKEND

    bool

    default y if IS_A_BACKEND = "y" || IS_A_BACKEND = "Y"

config BACKEND_ARCH

    string

    option env="CT_BACKEND_ARCH"

config BACKEND_KERNEL

    string

    option env="CT_BACKEND_KERNEL"

config BACKEND_LIBC

    string

    option env="CT_BACKEND_LIBC"