binutils/binutils: warn if only gold is selected

gold is not capable of building glibc/eglibc. See this thread:
http://sourceware.org/ml/crossgcc/2011-04/msg00010.html

Reported-by: Bill Pringlemeir <bpringle@sympatico.ca>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # m68k specific configuration file

 # depends on EXPERIMENTAL

 config ARCH_m68k

     select ARCH_SUPPORTS_32

     select ARCH_DEFAULT_32

     select ARCH_DEFAULT_BE

     select ARCH_SUPPORT_CPU

      help

       The m68k architecture