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>

 # s390 specific config options

 ## depends on EXPERIMENTAL

 ##

 ## select ARCH_SUPPORTS_32

 ## select ARCH_SUPPORTS_64

 ## select ARCH_DEFAULT_32

 ## select ARCH_USE_MMU

 ## select ARCH_SUPPORTS_WITH_FPU

 ##

 ## help Defined by IBM in:

 ## help     32-bit ESA/390       : http://publibfp.boulder.ibm.com/cgi-bin/bookmgr/BOOKS/dz9ar008/CCONTENTS

 ## help     64-bit z/Architecure : http://publibfp.boulder.ibm.com/cgi-bin/bookmgr/BOOKS/dz9zr002/CCONTENTS