Should not compile 3DNOW code for x86_64

Build for x86_64 should not compile 3DNOW as this code is not available in this environment.

Runtime detection of CPU features does not even look for 3DNOW support so that flag is not set in the features flag, making all 'if (features & MMX3DNOW)' code inaccessible (dead). Why use RAM and cache space for code that can never be executed? See detectProcessorFeatures() for x86_64 runtime detection.

The configuration script should consider the -no-3dnow switch to be implied when building for x86_64 environments. This prevents the definition of the QT_HAVE_3DNOW compiler def.

The definition of QT_HAVE_MMXEXT is inappropriate too, but there's no config switch for that one.