qemux86-64: build for x86-64-v3 (2013 Haswell and later) rather than Core 2 from 2006

This allows us to
- test those more recent instruction sets (AVX, AVX2, BMI1, BMI2, F16C, FMA, LZCNT, MOVBE, XSAVE)
- benefit from improved performance across the stack both in kvm-driven system emulation and when running
on real silicon.
For example, glibc:
https://www.phoronix.com/news/Glibc-strcasecmp-AVX2-EVEX

v4 level is adding AVX-512, which is far less established, particularly Intel has famously backtracked
from supporting it in Alder Lake/Raport Lake client CPUs and AMD has only implemented it in very recent Zen4 products:
https://www.phoronix.com/news/GCC-11-x86-64-Feature-Levels



Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>