C++: Add generic version of Float32xN for auto-vectorization

 - Define only RAYS_CPP_VECTORIZE to produce SSE auto-vectorization code.
 - Define RAYS_CPP_VECTORIZE and RAYS_CPP_VECTORIZE8 to produce AVX/SSE auto-vectorization code.

 - For GCC 4.8, these generic codes with any auto vectorization will generate always bad code.
 - For MSVC++2013, generic Float32x4 with SSE auto vectorization generates good code. But Float32x8 with AVX is not so good.