[libomptarget] Set runpath on libomptarget, use that to drop LD_LIBRARY_PATH from test runner

Using rpath instead of LD_LIBRARY_PATH to find libomp.so and
libomptarget.so lets one rerun the already built test executables without
setting environment variables and removes the risk of the test runner picking
up different libraries to the developer debugging the failure.

rpath usually means runpath, which is not transitive, so set runpath on
libomptarget itself so that it can find the plugins located next to it,
spelled $ORIGIN. This provides sufficient functionality to drop D102043

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D109071