Replace LZCNT with BSR instruction in hdr_histogram

[SeverinLeonhardt]

The LZCNT instruction produced by __lzcnt on Windows is not available
on many CPUs in use today. BSR is a safe replacement requiring only
minimal additional work.

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[mpenick]

@SeverinLeonhardt Thanks for researching and getting a pull request together. I just had one issue compiling on Windows, otherwise, LGTM.

[mpenick]

This shouldn't be const because it's reused for the next BSR.

[mpenick]

I noticed another issue too that's unrelated to the changes you made. For GCC-based compilers __builtin_clzll(0) is undefined so the code needs to check the value x for 0 (and return 0).

[SeverinLeonhardt]

Updated the pull request as you've suggested.

[mpenick]

Thanks for updating. Looks good.