Implement SysnoSet for each architecture

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.

Start by locating the current-architecture SysnoSet implementation and the architecture modules exposed through syscalls::{x86,arm,mips,...}::*. Read how those modules define their syscall APIs, then ensure SysnoSet is available for each exposed architecture; done means every architecture has the same const API without relying on const trait methods.

Written by the indexing model from the issue text.