[openmp] [test] Fix prepending config.library_dir to LD_LIBRARY_PATH

Fix the LD_LIBRARY_PATH prepending order to make sure that
config.library_path ends up before any potentially-system directories
(e.g. config.hwloc_library_dir).  This makes sure that we are testing
against the just-built openmp libraries rather than the version that is
already installed.

Also rename the function to `prepend_*` to make it clearer what it
actually does.

https://github.com/llvm/llvm-project/issues/56821
Differential Revision: https://reviews.llvm.org/D130825