Implement SysnoSet for each architecture

[jasonwhite]

Currently, SysnoSet is only implemented for the current architecture. It would be nice to round out the API and include it in all of the architectures exposed via syscalls::{x86,arm,mips,...}::*.

However, to do this, we'll have to generate separate implementations for each architecture. I looked into making a trait to reduce code duplication, but since most of the SysnoSet API is const, there is no way to call trait methods because they can't be const. Thus, I think the whole SysnoSet implementation will need to be wrapped in a macro to stamp out the implementation for each architecture.

Or, we can just wait for const in traits to be stabilized.