To be used in the next commit.
According to compiler explorer, __builtin_clz is very widely available,
and it barely makes sense to provide a fallback. clang also eats this
(and identifies at least as GCC 4). Actually, there's doubt that a fast
log2 implementation is needed at all (I guess UTF-8 parsing needs it,
but something UTF-8-specific would probably make it faster than using
log2). So the fallback is just something naive.
7510ed6f68