restrict availability on 32-bit platforms
restrict availability on 32-bit platforms

Summary:
Restrict F14 availability (and use the fallback version) on 32-bit platforms
until we audit the code, removing assumptions such as `size_t` is 64-bit and
finding a mixer that does not require `__int128`.

Reviewed By: nbronson

Differential Revision: D8525351

fbshipit-source-id: 54fce48fe94235c8525074216426cf254db26b4e

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly