autoPatchelfHook: Add addAutoPatchelfSearchPath

This function is useful if autoPatchelf is invoked during some of the
phases of a build and allows to add arbitrary shared objects to the
search path.

So far the same functionality was in autoPatchelf itself, but not
available as a separate function, so when adding shared objects to the
dependency cache one would have to do so manually.

The function also has the --no-recurse flag, which prevents recursing
into subdirectories.

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