Add sv_onexec_old() sysent hook for exec event

Unlike sv_onexec(), it is called from the old (pre-exec) sysentvec structure.
The old vmspace for the process is still intact during the call.

Reviewed by: dchagin, markj
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D30987