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

compiler/rustc_target/src/spec/targets/wasm32_unknown_emscripten.rs

@@ -5,10 +5,7 @@ use crate::spec::{
 pub fn target() -> Target {
     // Reset flags for non-Em flavors back to empty to satisfy sanity checking tests.
     let pre_link_args = LinkArgs::new();
-    let post_link_args = TargetOptions::link_args(
-        LinkerFlavor::EmCc,
-        &["-sABORTING_MALLOC=0", "-Wl,--fatal-warnings"],
-    );
+    let post_link_args = TargetOptions::link_args(LinkerFlavor::EmCc, &["-sABORTING_MALLOC=0"]);
 
     let opts = TargetOptions {
         os: "emscripten".into(),