arm64: Create a Linux view of the ID registers
When adding support for new hardware extensions we may not want to
enable support for the FreeBSD and Linux ABIs at the same time. To
support this split the Linux ID register and hwcaps so they can be
configured separately.
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D42372
(cherry picked from commit e6dbc99d47ddb254d75822817592bb82b5ce4d97)