AT_HWCAP support on aarch64

[Jan Beich]

armv6 and armv7 recently gained support to check NEON or VFP* at runtime.
However, some ports want to take advantage of AES, CRC32, SHA1, SHA256
acceleration on aarch64 as well.

Do you plan to support any HWCAP_* for AT_HWCAP on aarch64?

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=223016
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=223019 (ditto Chromium)
http://elixir.free-electrons.com/linux/latest/source/arch/arm64/include/uapi/asm/hwcap.h