It's often useful to be able to get the OS specifc raw values out
of a TimeStamp when interacting with other OS specific things. For
example, this lets us produce timestamps that are compatible with perf
or ETW.

This adds explicitly named per OS functions to do so. These should be
sufficiently unpleasant to use and clear in their semantics that it
should avoid any misuse.

Differential Revision: https://phabricator.services.mozilla.com/D177267