Update int8_simd.h (#13859)

Update int8_simd.h (#13859)

Summary:
If we use clang with sse4 support, we will have the function redefinition
error between [1] and [2]. This patch try to add some checkings to fix this
problem.

I just turn on USE_NATIVE_ARCH with clang, then I hit the redefinition error.

[1]
caffe2/operators/quantized/int8_simd.h
[2]
third_party/gemmlowp/gemmlowp/fixedpoint/fixedpoint_sse.h
Pull Request resolved: https://github.com/pytorch/pytorch/pull/13859

Differential Revision: D13095694

Pulled By: ezyang

fbshipit-source-id: c65166e4d5a04bb54e2b82c52740af00116ccb0d