[OpenMP] Add missing -L to libomptarget tests

Without this patch, if an incompatible libomptarget.so is present in a
system directory, such as /usr/lib64, check-openmp fails many
libomptarget tests with linking errors.  The problem appears to have
started at D129875, which landed as dc52712a.  This patch extends
the libomptarget test suite config with a -L for the current build
directory of libomptarget.so.

Reviewed By: jhuber6, JonChesterfield

Differential Revision: https://reviews.llvm.org/D149391