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.