Thread switching and stepping bug

[Andrew Cagney]

> Jonathan,
> 
> Note that my patch provides the generic PREPARE_TO_PROCEED functionality. You'll have to change your tdep file to use generic_prepare_to_proceed().  If your target is already multi-arched, it is a one line change in your {target}_gdbarch_init() function.  It would look like this:
> 
>   set_gdbarch_prepare_to_proceed (gdbarch, generic_prepare_to_proceed);
> 
> Give that a shot.

Perhaphs the generic version should be enabled by default?

While this is definitly a departure from the norm - retain default 
behavour even when broken - I suspect it may still be better since 
enabling this fixes a bug.

	Andrew