Restore -O3 settings for AVX targets for all platforms in LHCbMath. Fixes -O0 builds.
It appears from the nightlies that the AVX targets in LHCbMath, that use VectorClass, do not build properly under -O0. See
Previously -O3 was forced on all targets, for all platforms. I removed this when -O3 became the default without realising this detail. This MR just restores the -O3 setting for the AVX targets.