Some fixes to #[cfg] regarding Wasi

[tomaka]

I've been experimenting compiling and using libp2p for the wasm32-wasi target, and unfortunately our #[cfg] blocks are a bit weird regarding this platform.

In libp2p-core and libp2p-noise, when compiling for wasm, we opt-out from the ring library because the library doesn't support the wasm target. This is currently detected with target_os = "unknown" or any(target_os = "emscripten", target_os = "unknown").
Considering that the problem comes from ring containing inline assembly, which is an architecture-related problem, I changed that to target_arch = "wasm32".

At the crate root, we opt-out from crates such as libp2p-tcp and libp2p-mdns because they require the operating system to support TCP/UDP sockets. Similarly, this is currently detected with any(target_os = "emscripten", target_os = "unknown").
But here, since this is about a requirement from the operating system, I simply added wasi to the list, which is now any(target_os = "emscripten", target_os = "wasi", target_os = "unknown").

I included an unrelated change: libp2p-uds is currently enabled if all(unix, not(any(target_os = "emscripten", target_os = "unknown")). Since target_os = "unknown" and unix can and will, by definition, never be true at the same time, I removed it and kept only all(unix, not(target_os = "emscripten")).

Unfortunately, libp2p-secio doesn't compile for wasi. It does currently compile for wasm32-unknown-unknown because we assume that unknown equals "the browser", which is a hack. wasi isn't really meant to be run from the browser, therefore we can't apply the same hack.
For this reason, I can't add a CI check for the changes in this PR without being more invasive, which I don't think is a good idea for a low tier platform such as wasi.