syscalls: fix missing SIGSYS for several ENOSYS errors
In particular, when the syscall number is too large, or when syscall is
dynamic. For that, add nosys_sysent structure to pass fake sysent to
syscall top code.
Reviewed by: dchagin, markj
Discussed with: jhb
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D41976