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>