dtrace: internal performance measurement support code

dtrace: internal performance measurement support code

The dt_perf provider implements a few probes that are used in performance
(or more accurately put, overhead) measurements.  It uses an ioctl()
interface to trigger N-count interations of invoking probes through
various mechanisms, and a probe to post the results back to userspace.
This code also adds an SDT probe in the DTrace kernel support code, just
to measure overhead for triggering trap based SDT probes.

Signed-off-by: Kris Van Hees <kris.van.hees@oracle.com>