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
Approved by: re (gjb)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29314

(cherry picked from commit 5be27cbf16c85ce95d21db57349f61494f851821)
(cherry picked from commit db50c8123469e3eab07e931019bf6d4df2d55c78)