arm64: Add Arm SPE thread mode support

Add support for the using the Arm Statistical Profiling Extension (SPE) in
the hardware tracing framework, hwt(4), thread mode.

SPE is an optional extension added in Armv8.1 to provide profiling of
software using randomised instruction sampling. Thread mode allows tracing
of a single thread, regardless of the CPU it is scheduled on.

Reviewed by: andrew
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D53738