sysentvec: add SV_SIGSYS flag

to allow ABIs to indicate that SIGSYS is needed. Mark all native
FreeBSD ABIs with the flag.

This implicitly marks Linux' ABIs as not delivering SIGSYS on invalid
syscall.

Reviewed by: dchagin, markj
Discussed with: jhb
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D41976