armv8crypto: Avoid overriding -march when compiling some source files

At least one downstream (CheriBSD) overrides -march here. Instead of
hard-coding the target, simply append the crypto extension to whatever's
already there.

No functional change intended.

Reviewed by: imp, andrew
MFC after: 1 week
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40537