Updated performance patch series

[anton.ivanov at cambridgegreys.com]

1. I have revised the patches to fallback to x86 throughout instead
of falling back to generics.
2. Symlinks have been dropped in favour of direct references for the
x86 includes where applicable.
3. We use x86 CPU Feature flags now so anyone who wants to try their
luck doing the alternatives replacement can reuse some of the x86 code.
4. The perf gap between using kernel functions and using glibc has
narrowed a bit, but it is still there.