linux(4): Modify sv_onexec hook to return an error.

Temporary add stubs to the Linux emulation layer which calls the existing hook.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D30911
MFC after: 2 weeks

(cherry picked from commit 5fd9cd53d256e08c601548c425bfcb3472f2d09b)