Add icalls to Mono.Runtime for getting the arch/OS.

I need this for a C# library I'm working on which attempts to implement the C11/C++11 atomic memory model. I cannot use things like `Environment.Is64BitProcess` because I need to know the actual architecture - it's not a matter of word size. For example, on x86, barriers are usually not needed for an atomic load, while on ARM, they are. And so on.

Whether the `mono-config` APIs should be public is debatable and I don't particularly care myself. I just marked them public in case anyone thought they would be useful.