Install as arch-independent

[matwey]

xternsor-python is header only library, so prefer arch-independed paths cmake.

References:

• https://github.com/xtensor-stack/xtl/commit/d877d94836aff4d0f727acf3eaab8f4880ecb625
• https://github.com/xtensor-stack/xtensor/commit/7738389861044c9618c7d59fb7602f7dddc1df7f