[FEATURE] Port Krita to xsimd

This commit replaces our Vc backend with xsimd, QuantStack's SIMD offering backing the xtensor project.

This requires the following:

• wholesale replacement of Vc with xsimd alternatives
• implementation of dispatching against the xsimd::generic architecture (i.e. Vc::ScalarImpl)
• implementation of rudimentary gather-scatter and integer rounding ops

This brings Arm support to our backend for Free(tm).

TODO

• Correct implementation of target architecture detection
• Vc's will not work on Android or on a cross-compiled architecture
• Simplification of the xsimd Vc-ported CMake macros
• A way to make the benchmarks use the highest available dispatchable instruction set

Test Plan

Build Krita with xsimd. Check that you can paint without glitches.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.