In #92581 the `LibomptargetUitls.cmake` helpers have been removed, but only uses of `libomptarget_say` were migrated. Migrate the remaining few warning and error messages so the `check-offload` target would not fail due to missing `libomptarget_warning_say`. While at it, update the `check-offload` unavailability message to say `check-offload` instead of `check-libomptarget`. Fixes #92581