databases/mysql-udf: enable PIC on arm64 and i386, for lld

Shared objects should be built as PIC, and lld enforces this by default.
Add aarch64 and i386 cases to the existing set of per-arch CFLAGS, and
remove the BROKEN_aarch64.

Approved by: portmgr (lld blanket)
Sponsored by: The FreeBSD Foundation