The TIME_MONOTONIC maps to POSIX's CLOCK_MONOTONIC, TIME_ACTIVE to
CLOCK_PROCESS_CPUTIME_ID, and TIME_THREAD_ACTIVE to
CLOCK_THREAD_CPUTIME_ID.
No Linux specific timer are added as extension.
Co-authored-by: Yonggang Luo <luoyonggang@gmail.com>
Reviewed-by: Paul Eggert <eggert@cs.ucla.edu>
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