linux(4): Export the AT_EXECFN depending on the process osreldata

AT_EXECFN has appeared in the 2.6.26 Linux kernel first time.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D39647
MFC after: 1 month

(cherry picked from commit 70eab81d6ffecf0ba4c7f19a8b6389178eda5259)