Fix the AT_EXECFD functionality.

If the mapped object is linked at specific address, we must obey it.
If AT_EXECFD is not used, only in-kernel ELF image activator needed to
keep the mapping address, since only binaries are linked at the fixed
address, and binaries are mapped by kernel in this case.

Reviewed by: emaste
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
X-Differential revision: https://reviews.freebsd.org/D10701