Initialize CLOCK_UPTIME for itmer events

Since we move to implement Linux's CLOCK_MONOTONIC with CLOCK_UPTIME, we
broke the some timer support for Linux. Fix this by initializing
CLOCK_UPTIME as a posix clock so we can use in that context.

PR: 292496
MFC After: 5 days
Fixes: 108de784513d
Sponsored by: Netflix
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D54746