Linux legacy syscalls

[justincormack]

On newer architectures, many syscalls are now officially legacy, eg mkdir is not a syscall, have to use mkdirat.

See http://www.openwall.com/lists/musl/2014/05/25/1 for Musl response.

Although does not yet apply to existing architectures expect arm64 support at some point. So wrap calls in conditionals, and also have a "no legacy" override for testing.