arm64: Add support for emulating CTR_EL0

Emulate CTR_EL0 when it is either different on different cores or there
is errata affecting the core that means we need to report a different
value.

This adds the kernel part of Arm Neoverse-N1 erratum 1542419. This
depends on the firmware implementing part of the workaround. This is
safe even if the firmware is missing the workaround as it is telling
userspace to perform a stronger cache sync operation than is required,
just on a page at a time rather than a single cache-line.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D47814