Reimplement the arm64 dtrace_gethrtime(), which provides the
high-resolution nanosecond timestamp used for the DTrace 'timestamp'
built-in variable. The new implementation uses the EL0 cycle
counter and frequency registers in ARMv8-A. This replaces a
previous implementation that relied on an instrumentation-safe
implementation of getnanotime(), which provided only timer
resolution.

MFC after: 3 days
Reviewed by: andrew, bsdimp (older version)
Useful comments appreciated: jrtc27, emaste