[cmake] Allow to configure the linker used.

Summary: Now default to lld as it is even significantly faster than gold and packaged pretty much everywhere, except when a plateform file is explicitely specified in which case it uses the linker provide by the plateform.

Test Plan:

ninja all check-all

Cross compile OSX, ARM and Win64 to make sure they work properly.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8640