[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 dc52712.  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

openmp/libomptarget/test/lit.cfg

@@ -64,7 +64,8 @@ config.test_format = lit.formats.ShTest()
 # compiler flags
 config.test_flags = " -I " + config.test_source_root + \
     " -I " + config.omp_header_directory + \
-    " -L " + config.library_dir;
+    " -L " + config.library_dir + \
+    " -L " + config.llvm_lib_directory
 
 if config.omp_host_rtl_directory:
     config.test_flags = config.test_flags + " -L " + \