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.

compiler/rustc_target/src/spec/base/wasm.rs

@@ -38,9 +38,6 @@ pub fn options() -> TargetOptions {
                 // supposed to be imported and have all other symbols generate errors if
                 // they remain undefined.
                 concat!($prefix, "--allow-undefined"),
-                // Rust code should never have warnings, and warnings are often
-                // indicative of bugs, let's prevent them.
-                concat!($prefix, "--fatal-warnings"),
                 // LLD only implements C++-like demangling, which doesn't match our own
                 // mangling scheme. Tell LLD to not demangle anything and leave it up to
                 // us to demangle these symbols later. Currently rustc does not perform