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