gdb: hoist target_async_permitted checks into target.c

This commit moves the target_async_permitted check out of each targets
::can_async_p method and into the target_can_async_p wrapper function.

I've left some asserts in the two ::can_async_p methods that I
changed, which will hopefully catch any direct calls to these methods
that might be added in the future.

There should be no user visible changes after this commit.

gdb/target.c

@@ -399,6 +399,8 @@ target_can_async_p ()
 bool
 target_can_async_p (struct target_ops *target)
 {
+  if (!target_async_permitted)
+    return false;
   return target->can_async_p ();
 }