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