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.

Submitted by: andrew
MFC after: 1 week
Sponsored by: The FreeBSD Foundation