Detect SSE2 support

Compilation on an older 32-bit Athlon XP chip fails with the error
"./Utils/AEUtil.h:50:12: error: '__m128i' does not name a type"
This is because the __m128i type is only available on SSE2 platforms.
Modify the preprocessor logic to detect SSE and SSE2 support separately.

The "emmintrin.h" header should only be included on SSE2 platforms as
well.