In FreeBSD 10.0, the _umtx_op syscall was changed to allow sleeping on
any supported clock, but the default clock was switched from a monotonic
clock to CLOCK_REALTIME.

Prior to 10.0, the __umtx_op_wait* functions ignored the fourth argument
to _umtx_op (uaddr1), expected the fifth argument (uaddr2) to be a
struct timespec pointer, and used a monotonic clock (nanouptime(9)) for
timeout calculations.

Since 10.0, if callers want a clock other than CLOCK_REALTIME, they must
call _umtx_op with uaddr1 set to a value greater than sizeof(struct
timespec), and with uaddr2 as pointer to a struct _umtx_time, rather
than a timespec. Callers can set the _clockid field of the struct
_umtx_time to request the clock they want.

The relevant FreeBSD commit:
    https://svnweb.freebsd.org/base?view=revision&revision=232144

Fixes #17168

Change-Id: I3dd7b32b683622b8d7b4a6a8f9eb56401bed6bdf
Reviewed-on: https://go-review.googlesource.com/30154Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-on: https://go-review.googlesource.com/31269