Provide macros to get the WASI and WASI SDK versions

[brettcannon]

Brought up on Zulip#wasi > How to get the Wasm, WASI, & SDK version via C macros, having a way to get the compile-time values of the WASI version (e.g. 0.2) as well as the WASI SDK version (e.g. 29) would be great. I bring this up in the scope of CPython as there currently isn't a way to know these details from within CPython itself; see https://github.com/python/cpython/blob/main/Tools/wasm/README.md#detecting-webassembly-builds and how there is very little details about a WASI build compared to Emscripten.

This will also come into play when I attempt to get a Python Enhancement Proposal for wheels for WASI. There needs to be a way to get the relevant information that would be embedded in the file name#wasi > Platform tags for packages targeting WASI, and this is what's missing (__wasm32__ and __wasm64__ should be enough for CPU architecture).

[alexcrichton]

In #703 I'm proposing to use wasi/version.h as a wasi-libc-specific header file which defines __wasip{1,2,3}__. I'm not planning on just yet revealing the N in 0.2.N in the header, but that can be added in the future.

For the wasi-sdk version itself I think that's reasonable to export, but that'll require external configuration with a new cmake option. I'll work on getting that in next.