fix incorrect type annotations for coredata compile/link args

These make no sense as single strings.

mesonbuild/coredata.py

@@ -757,10 +757,10 @@ class CoreData:
             raise type(e)(('Validation failed for option %s: ' % option_name) + str(e)) \
                 .with_traceback(sys.exc_info()[2])
 
-    def get_external_args(self, for_machine: MachineChoice, lang: str) -> T.Union[str, T.List[str]]:
+    def get_external_args(self, for_machine: MachineChoice, lang: str) -> T.List[str]:
         return self.options[OptionKey('args', machine=for_machine, lang=lang)].value
 
-    def get_external_link_args(self, for_machine: MachineChoice, lang: str) -> T.Union[str, T.List[str]]:
+    def get_external_link_args(self, for_machine: MachineChoice, lang: str) -> T.List[str]:
         return self.options[OptionKey('link_args', machine=for_machine, lang=lang)].value
 
     def update_project_options(self, options: 'MutableKeyedOptionDictType') -> None: