[libc++] Replace uses of `__libcpp_ctz` by `__countr_zero`

[ldionne]

For a follow-up patch, we should replace uses of __libcpp_ctz in the codebase by calls to __countr_zero (there's only a few). This will eventually allow us to clean up __libcpp_ctz altogether.

Originally posted by @ldionne in #122641 (comment)