xen/arm64: add sys/arm64/include/xen/synch_bitops.h

[ehem_freebsd_m5p.com]

Summary

These are used when accessing data shared between Xen domains. Initial
testing suggests FreeBSD's atomics are suitable for this, but there is a
bit of uncertainty.

If the atomics are suitable for more platforms, then this header could
be used as architecture-independent. Whereas the existing versions are
very much specialized for their respective architectures.

This was broken off of a "xen/arm64: add xen platform" commit. This
portion seemed a rather distinct part of adding aarch64 support.

Submitted by: Elliott Mitchell <[e-mail]>
Original implementation: Julien Grall <[e-mail]>, 2014-01-13 17:40:58

[ehem_freebsd_m5p.com]

Adjusting several functions to be u_int as that seems more appropriate here (potentially reducing warnings if they get enabled).

[ehem_freebsd_m5p.com]

With D34153 making genuine atomics always available, now it is better to convert the Xen code to use the core atomics instead.