[Libomptarget] Update CMake messages if the tests aren't build

Summary:
These messages have been wrong for quite some time. Update them to be
more descriptive of why the tests weren't built.

Author: jhuber6

openmp/libomptarget/plugins/amdgpu/CMakeLists.txt

@@ -117,7 +117,6 @@ if (LIBOMPTARGET_FOUND_AMDGPU_GPU OR LIBOMPTARGET_FORCE_AMDGPU_TESTS)
   list(APPEND LIBOMPTARGET_TESTED_PLUGINS "omptarget.rtl.amdgpu")
   set(LIBOMPTARGET_TESTED_PLUGINS "${LIBOMPTARGET_TESTED_PLUGINS}" PARENT_SCOPE)
 else()
-  libomptarget_say("Not generating amdgcn test targets as libhsa is not linkable")
+  libomptarget_say("Not generating AMDGPU tests, no supported devices detected. Use 'LIBOMPTARGET_FORCE_AMDGPU_TESTS' to override.")
   return()
 endif()
-

openmp/libomptarget/plugins/cuda/CMakeLists.txt

@@ -72,5 +72,5 @@ if (LIBOMPTARGET_FOUND_NVIDIA_GPU OR LIBOMPTARGET_FORCE_NVIDIA_TESTS)
   list(APPEND LIBOMPTARGET_TESTED_PLUGINS "omptarget.rtl.cuda")
   set(LIBOMPTARGET_TESTED_PLUGINS "${LIBOMPTARGET_TESTED_PLUGINS}" PARENT_SCOPE)
 else()
-  libomptarget_say("Disabling tests using CUDA plugin as cuda may not be available")
+  libomptarget_say("Not generating NVIDIA tests, no supported devices detected. Use 'LIBOMPTARGET_FORCE_NVIDIA_TESTS' to override.")
 endif()