[Patch] Use __builtin_clz* for __lg

[Paolo Carlini]

Hi,

today, while working on something else, I noticed that we weren't
exploiting the various __builtin_clz* for our internal integer log2.
Seems a good idea to me, but please double check... Also, for the
generic case, I'm seeing a slightly tighter loop with the below tweak
(actually, that is apparently the most widespread way of implementing it).

Tested x86_64-linux.

Paolo.

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