cmake: fall back to march=corei7 on old compiler

In case corei7-avx is not supported corei7 is still the better choice
than core2.

Signed-off-by: Matthias Kretz <kretz@kde.org>
OptimizeForArchitecture: AMD 14h and 16h support

use btver2 or btver1 for AMD 16h / 14h CPUs

This fixes #620

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