565961bbf0
Remove --fatal-warnings on wasm targets These were added with good intentions, but a recent change in LLVM 18 emits a warning while examining .rmeta sections in .rlib files. Since this flag is a nice-to-have and users can update their LLVM linker independently of rustc's LLVM version, we can just omit the flag. See [this comment on wasm targets' uses of `--fatal-warnings`](https://github.com/llvm/llvm-project/pull/78658#issuecomment-1906651390).
rustc_target contains some very low-level details that are
specific to different compilation targets and so forth.
For more information about how rustc works, see the rustc dev guide.