windows: drop nanosleep and clock_gettime

windows: drop nanosleep and clock_gettime

Cygwin and msys2 now provide nanosleep and clock_gettime, so fio no
longer needs to implement them. The presence of our implementations was
triggering build failures:

https://github.com/axboe/fio/actions/runs/15828051168

Since fio no longer provides clock_gettime, stop unconditionally setting
clock_gettime and clock_monotonic to yes on Windows and start detectinga
these features at build time. These two features are successfully
detected by our configure script:

https://github.com/vincentkfu/fio/actions/runs/15832278184

Signed-off-by: Vincent Fu <vincent.fu@samsung.com>