Adjust target feature instructions for Wasm threading

[PgBiel]

Based on findings from godot-rust/gdext#1105 and godot-rust/gdext#1107, I propose:

1. Suggesting removing -Ctarget-features=+atomics when compiling a nothreads build, to ensure std is compiled without multi-threading (using polyfills for threading APIs).
2. Suggesting not adding the +mutable-globals target feature anymore in any wasm builds, as it appears to be stable and enabled by default now, per https://doc.rust-lang.org/1.85.1/rustc/platform-support/wasm32-unknown-unknown.html#enabled-webassembly-features
3. Tentatively suggesting removing the +bulk-memory target feature as well. In principle, some error messages from emscripten suggest to me that +atomics is sufficient here, as removing this flag made my rather simple test project (with async signals for fancier testing) work all the same. However, let's keep an eye on this in case someone has a problem with this.
4. Updating some minor things

[Bromeon]

Thanks a lot! Good to know that the usage of flags gets simpler with more recent emcc/rustc versions 👍

[Bromeon]

Thanks!