util/u_thread: Check for pthread_getcpuclockid.

pthread_getcpuclockid is not available on macOS.
Suggested-by: Eric Engestrom <eric.engestrom@intel.com>
Fixes: 4913215d

 ("util/u_thread: don't restrict u_thread_get_time_nano() to __linux__")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>