Alias ConcurrentHashMapSIMD to non-SIMD impl when the former isn't supported
Alias ConcurrentHashMapSIMD to non-SIMD impl when the former isn't supported

Summary:
Alias `ConcurrentHashMapSIMD` to `ConcurrentHashMap` when SSE4.2
isn't available.

Reviewed By: yfeldblum

Differential Revision: D37910670

fbshipit-source-id: 574ce181c738d39615a7d2682dcdb3b2ab6e9b31

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