gdb: add asserts in target.c for target_async_permitted

gdb: add asserts in target.c for target_async_permitted

The target_async_permitted flag allows a user to override whether a
target can act in async mode or not.  In previous commits I have moved
the checking of this flag out of the various ::can_async_p methods and
into the common target.c code.

In this commit I will add some additional assertions into
target_is_async_p and target_async.  The rules these assertions are
checking are:

  1. A target that returns false for target_can_async_p should never
  become "async enabled", and so ::is_async_p should always return
  false.  This is being checked in target_is_async_p.

  2. GDB should never try to enable async mode for a target that
  returns false for target_can_async_p, this is checked in
  target_async.

There are a few places where we call the ::is_async_p method directly,
in these cases we will obviously not pass through the assert in
target_is_async_p, however, there are also plenty of places where we
do call target_is_async_p so if GDB starts to misbehave we should
catch it quickly enough.

There should be no user visible changes after this commit.