binutils/binutils: add option to enable gold

gold is a new, optimised, multi-threaded linker with support
for plugins.

Add support for gold starting with binutils 2.21. Although 2.20
also had gold, the configure flags have changed, and supporting
2.20 would be a mess in the code.

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

 reporter_name="Ioannis E. VENETIS"

 reporter_url="http://sourceware.org/ml/crossgcc/2009-11/msg00109.html"

 reporter_comment=""