Import the Hardware Trace (HWT) framework.

The HWT framework provides infrastructure for hardware-assisted tracing. It
collects detailed information about software execution and records it as
"events" in highly compressed format into DRAM. The events cover information
about control flow changes of a program, whether branches taken or not,
exceptions taken, timing information, cycles elapsed and more. This allows
to reconstruct entire program flow of a given application.

This comes with separate machine-dependent tracing backends for trace
collection, trace decoder libraries and an instrumentation tool.

Reviewed by: kib (sys/kern bits)
Sponsored by: UKRI
Differential Revision: https://reviews.freebsd.org/D40466