libcrypto: Copy the arm64 header when building asm

It may be needed when it's updated so is best to keep in sync with the
assembly files.

Reviewed by: emaste
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D41938

(cherry picked from commit c97a82d4a4a0288ed2a456f4ce41d57483724f17)