Move default is_linkable_target method up from BuildTarget to Target

BuildTarget.link() assumes that it can call is_linkable_target() on any
objects which are a subclass of Target

mesonbuild/build.py

@@ -363,6 +363,8 @@ a hard error in the future.''' % name)
             result[k] = v
         return result
 
+    def is_linkable_target(self):
+        return False
 
 class BuildTarget(Target):
     known_kwargs = known_build_target_kwargs
@@ -1104,9 +1106,6 @@ You probably should put it in link_with instead.''')
             return True
         return False
 
-    def is_linkable_target(self):
-        return False
-
     def check_module_linking(self):
         '''
         Warn if shared modules are linked with target: (link_with) #2865