Reimplemen FreeBSD/arm64 dtrace_gethrtime() to use the system timer.

Reimplemen FreeBSD/arm64 dtrace_gethrtime() to use the system timer.
dtrace_gethrtime() is the high-resolution nanosecond timestemp 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.

Approved by:	re (gjb)
Reviewed by:	andrew, bsdimp (older version)
Useful comments appreciated:	jrtc27, emaste
Differential Revision:	https://reviews.freebsd.org/D28723