Drop relative library path components before filtering against
system runpath, to avoid duplicate usr/lib via usr/lib/gcc/..
This allows workaround-libraries in usr/lib/host/gcc to be found
at runtime before the real ones in usr/lib, while finding the
real ones at linktime in usr/lib/gcc/.. first.
Such workarounds might be useful on AIX to enable SONAME
support for already installed libraries, fex libz.so might be
erroneously used at runtime, which breaks when supporting SONAME.