[OpenMP][Docs] Add documentation for using configuration files

We recently reverted a patch that automatically set the rpath on OpenMP
executables. This was used because the `libomptarget.so` library is only
expected to work with the same version of compiler that will be using
it. This patch adds some documentation for how to get similar behaviour
as before using a clang configuration file.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D147943