Linux: Add FUTEX_LOCK_PI2

Linux v5.14.0 introduced a new futex operation called FUTEX_LOCK_PI2.

This kernel feature can be used to implement
pthread_mutex_clocklock(MONOTONIC)/PI.

Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>