Ubuntu 18 updates

Ubuntu no longer supports 32 bit code easily so we have had to change the default code generation model to AMD64. There were also complexities associated with the fact that you can no longer use the latest gcc linkers to link anything but RIP relative labels for 64 bit code. This means that the old AVX code generator AVX32 is no longer usable under 64 bit Ubuntu and a new code generator AVX64 has been added.