MFC r319718: arm64: add ".arch armv8-a+crc" to allow use of crc instructions

With Clang 5.0 the .arch directive is required, otherwise Clang
complains "error: instruction requires: crc".

This was reported in D10499 but not added initially, because clang 3.8
available on a ref machine reported unknown directive. Clang 4.0 allows
but does not require the directive.

Sponsored by: The FreeBSD Foundation

MFC r317512:

armv8 has support for optional CRC32C instructions. This patch checks if they are
available and if that is true make use of them.
Thank you very much to Andrew Turner for providing help and review the patch!

Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D10499

armv8 has support for optional CRC32C instructions. This patch checks if they are
available and if that is true make use of them.
Thank you very much to Andrew Turner for providing help and review the patch!
Reviewed by: andrew
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D10499