Log Message:
graphics/qt6-qt3d: fix broken build.

Make AVX2 support on x86_64 optional (disabled by default), to prevent
PLIST fialures on older models.
Turn on SIMD (SSE2) support on i386 too, still to avoid PLIST issues.
This will likely break PLIST on CPUs before Pentium 4, but the use case
of qt6-qt3d on 486 -> Pentium III seems limited.