Bug 1679938 - Don't recurse the build into mfbt for projects that don't need it. r=firefox-build-system-reviewers,mhentges

While all toolkit and js-based projects make use of mfbt, some others,
like tools/crashreporter and tools/update-packaging, don't.

So instead of including mfbt from the top-level directory, include it
from the relevant project top-level mozbuilds.

This allows to remove the dependency on mfbt files in the hash for the
minidump-stackwalk and mar-tools toolchains.

Differential Revision: https://phabricator.services.mozilla.com/D98378