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>

 # Default test suite configuration

 # Default DejaGnu configuration

 DG_TOOLNAME = gcc

 DG_TARGET_HOSTNAME = 127.0.0.1

 DG_TARGET_USERNAME = root

 # Default tests

 DG_C_TESTS = 

 DG_CPP_TESTS =