arm64: fix incorrect HWCAP definitions

FreeBSD exports CPU features as bits in the AT_HWCAP and AT_HWCAP2
vectors via elf_aux_info(3). This interface is similar to getauxval(3)
on Linux, and for simplicity to consumers we try to maintain an
identical set of feature flags on arm64.

The first batch of AT_HWCAP flags were added in r350166, corresponding
to definitions that already existed in Linux. Unfortunately, one flag
was missed, and a portion of the values are shifted one bit to the right
as a result.

Add the missing definition for HWCAP_ASIMDHP, and adjust the affected
values to match their Linux counterparts.

Although this is an ABI-breaking change, there is no plan to provide
compat code for old binaries. An audit of our ports tree and other
software via Debian code search indicates that there are not yet any
consumers of this interface for FreeBSD/arm64.

Bump __FreeBSD_version to be on the safe side, in case compat code needs
to be added in the future.

Reviewed by: emaste, manu
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26329