adapt dynamic linker for new binutils versions that omit DT_RPATH

adapt dynamic linker for new binutils versions that omit DT_RPATH

the new DT_RUNPATH semantics for search order are always used, and
since binutils had always set both DT_RPATH and DT_RUNPATH when the
latter was used, processing only DT_RPATH worked fine. however, recent
binutils has stopped generating DT_RPATH when DT_RUNPATH is used,
which broke support for this feature completely.