[rfc] Swap out current when creating a new architecture

[Andrew Cagney]

> 
> Just a quick recap: what the old code was doing by referring to
> current_gdbarch was to avoid changing the ABI (which I infer from the
> bfd) if one of the other target specific commands were being used.  The
> patch below sets the abfd field in the info struct to exec_bfd before
> calling gdbarch_update_p, so that information on the current bfd is
> passed along, just as it is when gdbarch_update_p is called from
> set_gdbarch_from_file.
> 
> Is this an acceptable way of doing it?  I know I'm in the risky business
> of dealing with global pointers.  (I'm assuming a commit could go on
> both trunk and branch.)

Nice bug! I need to think about this some more though.

As for the branch, no need as I'm not committing my change to that.

Andrew