arm64: implement COMPAT_FREEBSD32 fill/set dbregs/fpregs

On FreeBSD/arm fill_fpregs, fill_dbregs are stubs that zero the reg
struct and return success. set_fpregs and set_dbregs do nothing and
return success.

Provide the same implementation for arm64 COMPAT_FREEBSD32.

Reviewed by: andrew
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29314