cmake: Refactor and extend architecture flags
cmake: Refactor and extend architecture flags

* Reduce code duplication with new helper macros.
* Add support for setting (GCC compatible) compiler flags for AVX-512
systems (Skylake, Cannonlake, Knights Landing).
Refs: gh-102

Signed-off-by: Matthias Kretz <372ea4eca737a13cd010ee806d4ce7b98ee71550@kde.org>

VcDevel/Vc,chr-engwer/Vc,VcDevel/Vc,chr-engwer/Vc,chr-engwer/Vc,VcDevel/Vc,VcDevel/Vc