Add an sfmt19937 benchmark
Add an sfmt19937 benchmark

Summary: `__gnu_cxx::sfmt19937` is 3.5x faster than `std::mt19937` according to the benchmark.

Reviewed By: yfeldblum

Differential Revision: D5393697

fbshipit-source-id: bc725838dc7f62d030c7e68bc6d229042df2b46c

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