Rollup merge of #65151 - tmandry:revert-emscripten-upgrade, r=tmandry

Revert #63649 - "Upgrade Emscripten targets to use upstream LLVM backend"

This change caused the runtime of the linux-asmjs builder to nearly double from 2+ hours to about 4 hours, which happens to be the bors timeout. (It made it in barely under 4 hours when it was merged.) This is causing timeouts on all new changes.

This reverts commit 7870050796, reversing
changes made to 2e7244807a.

src/librustc_codegen_llvm/llvm_util.rs

@@ -257,7 +257,8 @@ pub fn target_feature_whitelist(sess: &Session)
         "hexagon" => HEXAGON_WHITELIST,
         "mips" | "mips64" => MIPS_WHITELIST,
         "powerpc" | "powerpc64" => POWERPC_WHITELIST,
-        "wasm32" => WASM_WHITELIST,
+        // wasm32 on emscripten does not support these target features
+        "wasm32" if !sess.target.target.options.is_like_emscripten => WASM_WHITELIST,
         _ => &[],
     }
 }