add WebAssembly support

[williballenthin]

This PR makes some minor, non-breaking changes that enable building Rust libraries that depend on zydis-rs to WebAssembly. Its really cool how little was needed to make this happen!

To make use of this PR, zyantific/zycore-c#40 and zyantific/zydis#277 need to be merged. I didn't attempt any of these changes here, since I'm not familiar enough with your release process. However, I have a temporary branch that you can reference from Cargo.toml: nightly/wasm.

In summary, this PR does two things:

• introduce features for nolibc and wasm, and
• tweaks a datatype to fit within the wasm32 isize range

Unfortunately, building zydis-rs for WebAssembly is not as trivial as just invoking wasm-pack, but it can be done. Today, the build.rs script invokes the system C/C++ compilers and linker on the zydis-c and zycore libraries, but these tools

1. usually don't know how to build and link WebAssembly object files, and
2. the gcc/clang-generated object files may not be ABI-compatible with rustc-generated object files.

However, I found that the trick from here to use the Zig compiler instead of gcc/clang to work perfectly. I've updated my demo project zydis-wasm to show this in action.

[th0rex]

Thanks, LGTM. I think I'll merge these changes now, instead of waiting for the zydis/zycore PRs to be merged, they are independent anyways. I think updating zydis-c should be relatively straight forward, there shouldn't have been any breaking changes (yet), but I'll do that in a follow up commit (or feel free to do it if you want to/if your branch already works).