Bug 1618766 - Properly find and use MT on Windows cross-builds. r=froydnj

- Remove the separate option() for MT, because it dates back from when
  we needed `MT` not being an absolute path, but that hasn't been true
  since bug 1290040.

- Extend what was done in bug 1617794 to MT, although the long term move
  is to not rely on MT at all.

- Patch leftovers from bug 1613799.

Differential Revision: https://phabricator.services.mozilla.com/D64712

--HG--
extra : moz-landing-system : lando