Add libdtrace support for arm64 USDT probes.

arm64 is still lacking a fasttrap implementation, which is required to
actually enable userland probes, but this at least allows USDT probes to
be linked into userland applications.

Submitted by: Klaus Küchemann <maciphone2@googlemail.com> (original)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D22360