arm64: check for CRC32 support via HWCAP

Doing it this way eliminates the assumption about homogeneous support
for the feature, since HWCAP values are only set if support is present
on all CPUs.

Reviewed by: tuexen, markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26032