gdb: introduce a new overload of target_can_async_p

gdb: introduce a new overload of target_can_async_p

There are a few places where we call the target_ops::can_async_p
member function directly, instead of using the target_can_async_p
wrapper.

In some of these places this is because we need to ask before the
target has been pushed, and in another location (in target.c) it seems
unnecessary to go through the wrapper when we are already in target.c
code.

However, in the next commit I'd like to hoist some common checks out
of target specific code into target.c.  To achieve this, in this
commit, I introduce a new overload of target_can_async_p which takes a
target_ops pointer, and calls the ::can_async_p method directly.  I
then make use of the new overload where appropriate.

There should be no user visible changes after this commit.