[libc] Use __builtin_clz to find leading 1 in hypot

This is an optimization that using a single CPU instruction on supported
architectures (amd64 and aarch64, but possibly others) to replace what was
previously an iterative look-up-table algorithm.

Originally I suggested using inline assembly for this in
https://reviews.llvm.org/D117584.

Reviewed By: lntue, sivachandra

Differential Revision: https://reviews.llvm.org/D117684