nptl: Add CLOCK_MONOTONIC support for PI mutexes

Linux added FUTEX_LOCK_PI2 to support clock selection
(commit bf22a6976897977b0a3f1aeba6823c959fc4fdae).  With the new
flag we can now proper support CLOCK_MONOTONIC for
pthread_mutex_clocklock with Priority Inheritance.  If kernel
does not support, EINVAL is returned instead.

The difference is the futex operation will be issued and the kernel
will advertise the missing support (instead of hard-code error
return).

Checked on x86_64-linux-gnu and i686-linux-gnu on Linux 5.14, 5.11,
and 4.15.