Make it possible to execute PIE binaries that don't have an PT_INTERP segment

Make it possible to execute PIE binaries that don't have an PT_INTERP segment
(and therefore no interpreter).  Necessary to make self-relocating "static"
PIE working.

This will also allow execution of shared libraries that have the 'x' bit set.
Those shouldn't exist, but unfortunately people outside OpenBSD think
differently which means third-party software sometimes installs shared
libraries with the 'x' bit set.

ok kurt@, deraadt@