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>

 reporter_name="Julien DUSSER"

 reporter_url="http://sourceware.org/ml/crossgcc/2008-10/msg00054.html"

 reporter_comment="Soft-float for the 405 PowerPC"