Add C2X timespec_getres

ISO C2X adds a timespec_getres function alongside the C11
timespec_get, with functionality similar to that of POSIX clock_getres
(including allowing a NULL pointer to be passed to the function).
Implement this function for glibc, similarly to the implementation of
timespec_get.

This includes a basic test like that of timespec_get, but no
documentation in the manual, given that TIME_UTC and timespec_get
aren't documented in the manual at all.  The handling of 64-bit time
follows that in timespec_get; people maintaining patch series for
64-bit time will need to update them accordingly (to export
__timespec_getres64, redirect calls in time.h and run the test for
_TIME_BITS=64).

Tested for x86_64 and x86, and (previous version; only testcase
differs) with build-many-glibcs.py.