This diff adds an USE_LD_GOLD option that attempts to use gold
instead of the default toolchain linker. It is ON by default since it
has a safe fallback path.
Note that it works when cross compiling too. The option is also
available with MinGw but supposedly has no effect since gold is
Building with debug on my machine (with ccache so linking time is more
real 1m28.916s user 1m46.125s sys 0m46.567s
real 1m5.641s user 1m21.068s sys 0m42.618s