rts/OSThreads: Fix reference clock of timedWaitCondition

Previously timedWaitCondition assumed that timeouts were referenced against CLOCK_MONOTONIC. This is wrong; by default pthread_cond_timedwait references against CLOCK_REALTIME, although this can be overridden using pthread_condattr_setclock.

Fix this and add support for using CLOCK_MONOTONIC whenever possible as it is more robust against system time changes and is likely cheaper to query.

Fixes #20144 (closed).