Add support for non-Neon ARM

[aaronfranke]

https://github.com/embree/embree/blob/master/common/sys/intrinsics.h

#if defined(__ARM_NEON)
#include "../simd/arm/emulation.h"
#else
#include <immintrin.h>
#endif

When compiling for Windows for ARM using MSVC, __ARM_NEON is not defined so it tries to #include <immintrin.h> which fails because this isn't x86, and if I force #include "../simd/arm/emulation.h" then it also fails because Embree uses the sse2neon library which does not support MSVC yet.

Would it be possible to allow Embree to run on ARM without Neon? Or maybe just add an option that replaces the SSE instructions with a software-only mode that doesn't depend on specific CPU instructions, so it will compile on any CPU?

At the very least, the above code snippet should be changed to detect if neither x86 nor ARM Neon is the case and error with a descriptive message such as "Only x86 and ARM Neon are supported".