This fixes a bug whereby the interrupt thread would immediately throw on
the Hurd due to ‘clock-nanosleep’ being passed a value it doesn’t
support, and thus throwing to EINVAL.
- fibers/interrupts.scm (with-interrupts/thread-cputime): Set ‘clockid’
to ‘CLOCK_MONOTONIC’ on the Hurd.
Fixes: guile/fibers#171
Reported-by: Yelninei