clock_gettime: Fix CLOCK_THREAD_CPUTIME_ID race
Use a spinlock section instead of a critical section to synchronize with
statclock(). Otherwise the CLOCK_THREAD_CPUTIME_ID clock can appear to
go backwards.
PR: 262273
Reviewed by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D34568