Set rpath on openmp executables

Openmp executables need to find libomp and libomptarget at runtime.
This currently requires LD_LIBRARY_PATH or the user to specify rpath. Change
that to set the expected location of the openmp libraries in the install tree.

Whether rpath means rpath or runpath is system dependent. The attached test
shows that the Wl,--disable-new-dtags control interacts correctly with this feature.

The implicit rpath field is appended to any user specified ones which is ideal.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D118493