[Performance] Check whether xsimd is properly integrated

[Ivorforce]

xsimd is compiled along xtensor, but i have not seen substantial speed improvements, whether it is enabled or not. Are simd operations actually run yet?

A former SIMD test project supposedly achieved up to 300x speed improvements over native Godot. While Godot speeds might have improved since then, this would be a great horizon to achieve.

[Ivorforce]

I have achieved substantial speedup through f7ff3f2. I now think it more likely that SIMD is actually running, but we still need the test.

[Ivorforce]

I think I have gathered enough evidence to conclude xsimd is properly integrated:

1. xtensor fails if XTENSOR_USE_XSIMD is defined but xsimd not linked.
2. SIMD- related cpp defines are properly defined.
3. When building with march=native, I can achieve ~2x improvements for specific types of operations (up to 550x improvement over GDScript total).