Summary:
Currently to_ascii_port_clzll() in ToAscii.h always returns 0 in x86. This switches to __builtin_clzll() backed by a polyfill.

Closes https://github.com/facebook/folly/issues/1566.

Pull Request resolved: https://github.com/facebook/folly/pull/1567

Reviewed By: luciang

Differential Revision: D28162903

Pulled By: yfeldblum

fbshipit-source-id: 1872cda7aa155b84086bd15ad6cd40475eeb0ac4