BZ#13926: Add __bswap_64 for non-GCC compilers

[Roland McGrath]

> How do you know that the non-GCC compiler in question supports
> unsigned long long type at all?  It is not a standard C89 type...
> Why are you using ul rather than ull suffixed constants?

Both good points, which I should have noticed.
The latter is a problem even in C99 compilers.
I retract my previous approval until these things are addressed.


Thanks,
Roland