[D145982] [libc++] Implement std::gcd using the binary version

[ldionne]

This issue tracks picking up https://reviews.llvm.org/D145982 from the Phabricator archive.

[AdvenamTacet]

I played a little with the benchmark from the Phabricator to test also bigger numbers in gcd. Performance gain is consistent.

But I think, we should add random tests to gcd test - 1000 times taking ranom numbers or numbers from a range and comparing with a naive implementation. Just because this implementation is prone to be bugged during a refactor.

[serge-sans-paille]

@AdvenamTacet : will do!