Thanks for the idea, and the PR!

To be useful, this would need a bit of tweaking: we can't assume that a digit is always `unsigned long` (in fact, I'd expect that to be rare on 64-bit non-Windows systems, where `unsigned long` typically has 64 bits, and `digit` should be using a 32-bit type), so we'd need to identify and use the most appropriate variant of clz/clzl/clzll somehow.

I think it would also make sense to move the detection of the existence of `__builtin_clz` and friends into the configuration machinery: these builtins aren't just restricted to GCC (clang supports them as well), and that would allow other platforms to provide their own substitutes by modifying `pyport.h`. Ideally, the configuration machinery #defines a HAVE_CLZ variable, and then in longobject.c we do an #ifdef HAVE_CLZ ...

We also need to trade-off the additional complication in the implementation against the benefits: though we do certainly care about the speed, speed at all costs isn't the target here; readability, portability and maintainability of the source counts, too. It'll probably be easier to weigh those factors once there's an implementation that addresses the above points.