arm64: Add an is64bit flag to user readable ID regs

To tell if a register is for 32 or 64 bit userspace add a flag to the
user reg struct. This will be used later to rework how the hwcaps are
generated.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D42370

(cherry picked from commit c2dc043c9757aad77bb1745e3b14dfa6b27e3fa4)