arm64: Read the CPU feature tunables once

Rather than reading the CPU tunables each time we enable a feature
just read them once on boot. Reading them early on non-boot CPUs
causes issues where they try to allocate memory before the core is set
up to do so. As they don't change it is safe to cache the state of
them.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D52578