[libcxx][libcxxabi] Fix build for OpenBSD (#92186)

- No indirect syscalls on OpenBSD. Instead there is a `futex` function
which issues a direct syscall.

- Monotonic clock is available despite the full POSIX suite of timers
not being available in its entirety.

  See https://lists.boost.org/boost-bugs/2015/07/41690.php and
  https://github.com/boostorg/log/commit/c98b1f459add14d5ce3e9e63e2469064601d7f71
  for a description of an analogous problem and fix for Boost.