arm64: fix struct l_sigaction_t layout

The definition was copied from amd64, but the layout of the struct
differs slightly between these platforms. This fixes spurious
unsupported sigaction flag 0xXXXXXXXX messages when executing some
Linux binaries on arm64.

Reviewed by: emaste
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D27460