Add -march=i586 to CXXFLAGS

This follows on from adding the same -march setting to oiio (cmake/platform.cmake). An arch of i586 is needed for the atomic cpu instructions that are now used from oiio. I believe the addition of using OIIO::atomic_int and using OIIO::atomic_ll to src/liboslexec/oslexec_pvt.h triggered this.