autoPatchelfHook: Only check PT_INTERP on execs

If the ELF file is not an executable, we do not get a PT_INTERP section,
because after all, it's a *shared* library.

So instead of checking for PT_INTERP (to avoid statically linked
executables) for all ELF files, we add another check to see if it's an
executable and *only* skip it when it is and there's no PT_INTERP.

Signed-off-by: aszlig <aszlig@nix.build>