Fix a completely wrong calculation in mach monotime_init_internal()

Fix a completely wrong calculation in mach monotime_init_internal()

Bug 1: We were purporting to calculate milliseconds per tick, when we
*should* have been computing ticks per millisecond.

Bug 2: Instead of computing either one of those, we were _actually_
computing femtoseconds per tick.

These two bugs covered for one another on x86 hardware, where 1 tick
== 1 nanosecond.  But on M1 OSX, 1 tick is about 41 nanoseconds,
causing surprising results.

Fixes bug 40684; bugfix on 0.3.3.1-alpha.