Fix for Bug 19851 (try 3) - Interlocked instruction support for ARM

[Colin Fowler]

I've only tested on ARM and wasn't comfortable enabling it for all other 
architectures (although I too see no reason why it wouldn't work).

Colin

Alexandre Julliard wrote:
> Colin Fowler <elethiomel at gmail.com> writes:
> 
>> @@ -396,6 +396,88 @@
>>                    "beq   $1,L0xchg_add\n\t"
>>                    "mb")
>>  
>> +
>> +#elif defined(__arm__)
>> +
>> +/* 
>> + * Similar to the Sparc, many ARM CPUs lack the necessary 
>> + * atomic instructions. Fall back in a similar way, but instead
>> + * use pthreads. This solution is generic enough to work on 
>> + * pretty much any CPU, but is still very fast thanks to 
>> + * Linux's NPTLS.
>> + */
> 
> Actually since it's generic there's no reason to put it in an __arm__
> ifdef, it could simply be the default fallback.
>