[offload] Define OPENMP_TEST*_FLAGS in standalone builds

Copy the `OPENMP_TEST_FLAGS` and `OPENMP_TEST_OPENMP_FLAGS` from openmp
for standalone builds, as required by the lit configs.

Author: mgorny

offload/CMakeLists.txt

@@ -283,6 +283,11 @@ if(OPENMP_STANDALONE_BUILD)
       ${LLVM_LIBRARY_DIRS}
     REQUIRED
   )
+
+  set(OPENMP_TEST_FLAGS "" CACHE STRING
+    "Extra compiler flags to send to the test compiler.")
+  set(OPENMP_TEST_OPENMP_FLAGS ${OPENMP_TEST_COMPILER_OPENMP_FLAGS} CACHE STRING
+    "OpenMP compiler flag to use for testing OpenMP runtime libraries.")
 endif()
 
 macro(pythonize_bool var)