build, win: make LTCG optional

Disables Link Time Code Generation by default. Adds ‘ltcg’ vcbuild
option to enable it. LTCG will be used by default by release and CI
builds.

PR-URL: https://github.com/nodejs/node/pull/21186


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>