Fix error due to using SSSE3 _mm_abs_epi32 when compiling for SSE2

This `abs` function doesn't seem to be used in the OIIO texture sampling or OSL noise code so this change should not slow them down.

I also noticed that there is no distinction made between SSE3 and (the unfortunately named) SSSE3. So someone with a SSE3 CPU and compiling with USE_SIMD=sse3 may not be able to run the binary if the CPU does not support SSSE3, which could be confusing.