arm64: Add Arm SPE support

Add support for the Arm Statistical Profiling Extension (SPE). This
is an optional extension added in Armv8.1 to provide profiling of
software using randomised instruction

arm64: Add Arm SPE support

Add support for the Arm Statistical Profiling Extension (SPE). This
is an optional extension added in Armv8.1 to provide profiling of
software using randomised instruction sampling.

This adds the initial driver, and attached it to the hardware tracing
framework, hwt(4), in CPU mode to allow tracing of all threads on
a given CPU.

(commit message by andrew@)

Co-authored-by: Sarah Walker <sarah.walker2@arm.com>
Co-authored-by: Andrew Turner <andrew@FreeBSD.org>
Reviewed by: andrew
Sponsored by: Arm Ltd
Sponsored by: The FreeBSD Foundation (early driver)
Differential Revision: https://reviews.freebsd.org/D46241

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