[CMAKE] Use gold as a linker when available

Summary:
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
ELF-only.

Building with debug on my machine (with ccache so linking time is more
relevant), before:

real    1m28.916s
user    1m46.125s
sys     0m46.567s

After:

real    1m5.641s
user    1m21.068s
sys     0m42.618s

Test Plan:
On Linux and OSX:

ninja all check

Run the Gitian builds twice and check the build is still deterministic.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5465