[Libomptarget] Fix the NVPTX Libomptarget test

Summary:
This was broken, we weren't adding these for the NVPTX tests.

Author: jhuber6

openmp/libomptarget/test/lit.cfg

@@ -114,7 +114,7 @@ else: # Unices
     config.test_flags += " -Wl,-rpath," + config.llvm_lib_directory
     if config.cuda_libdir:
         config.test_flags += " -Wl,-rpath," + config.cuda_libdir
-    if config.libomptarget_current_target.startswith('amdgcn'):
+    if config.libomptarget_current_target.startswith('nvptx'):
         config.test_flags += " --libomptarget-nvptx-bc-path=" + config.library_dir
     if config.libomptarget_current_target.endswith('-LTO'):
         config.test_flags += " -foffload-lto"