Fix support for CPU dispatching with GCC7 on older CPUs that do not support AVX.

This MR addresses some issues with the CPU dispatching used in the RICH when built with GCC7, that caused the binaries to be unusable on machines that did not support AVX. Fixed by disabling the automatic insertion of vzeroupper calls by the compiler in the dispatch compilations, using -mno-vzeroupper. Instead these calls manually call the method in a safe way.

Uses changes in LHCb!848 (merged).