arm64: parse HWCAP values using user_cpu_desc

The hard work of parsing fields per-CPU, handling heterogeneous
features, and excluding features from userspace is already done by
update_special_regs. We can build our set of HWCAPs from the result.

This exposed a small bug in update_special_regs, in which the
generated bitmask was not wide enough, and as a result some bits
weren't being exposed in user_cpu_desc. Fix this.

While here, adjust some formatting.

Reviewed by: andrew
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26069