PowerPC and MIPS do not have AtomicU64, but AtomicUsize is a more
portable type that can be used as a replacement.

[import_pr] From #3884

Differential Revision: https://phabricator.services.mozilla.com/D67365

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/ff701e82864569f328f9466ba5fdbfb0273354d6