Make rel-rpath.sh future safe (RPATH vs. RUNPATH)

patchelf tries to fix RPATH, however it has long been deprecated in
favour of RUNPATH. For now the host dso's have RPATH, however it could
change under the hood one day - so make sure we test of either.

This came to light because patchelf --set-rpath on Ubunti 13.10 (64bit)
ends up generating RUNPATH (unless --force-rpath used).

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Cc: Anton Kolesov <Anton.Kolesov@synopsys.com>
Cc: Simon Cook <simon.cook@embecosm.com>
Cc: Jeremy Bennett <jeremy.bennett@embecosm.com>