[MinGW] RFA: Make Stack Traces Work for Interpreted Code on Windows

[Bryce McKinlay]

Ranjit Mathew wrote:
>> I think you could just overload them with #defines in backtrace.h? 
>> Failing that, I think that adding a couple of #ifdef's in UnwindTraceFn 
>> is not as bad as the code duplication.
>>     
>
> That worked pretty nicely, thank you. It reduces unnecessary
> duplication of code too. The attached patch implements it and
> was tested using a crossed-native compiler for i686-pc-mingw32.
> I still get usable stack traces for compiled and interpreted
> code.
>
> OK?
>   

It occurs to me that you could now actually #define fallback_backtrace 
as _Unwind_Backtrace, and get rid of some #ifdefs in stacktrace.cc. Do 
you think that is worth doing? Otherwise, this is OK for trunk.

Bryce