[rfc] Swap out current when creating a new architecture

[Andrew Cagney]

>> For MIPS, the problem has so far been avoided by coding functions as:
>> 
>> if (set by user)
>> return user value;
>> else
>> return value from architecture;
>> 
> 
> 
>>From looking at the MIPS target just now I thought it would suffer from
> the same problem when it comes to guessing the ABI (since info.abfd is
> used if present).  But then I saw that the functions tied to the MIPS
> specific user-commands don't call gdbarch_update, they change
> current_gdbarch directly.  I'm going to try something similar.

For what it is worth.  I'd not be too worried about getting it perfect. 
   Multiple architecture support is still pretty much an accademic 
experiment.

Andrew