[issue29782] Use __builtin_clzl for bits_in_digit if available

[Niklas Fiekas]

Change by Niklas Fiekas <niklas.fiekas at backscattering.de>:


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pull_requests: +19946
pull_request: https://github.com/python/cpython/pull/20739

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue29782>
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