Auto merge of #65251 - tlively:emscripten-upstream-upgrade, r=tlively

Upgrade Emscripten targets to use upstream LLVM backend

 - Compatible with Emscripten 1.38.46-upstream or later upstream.
 - Refactors the Emscripten target spec to share code with other wasm
   targets.
 - Replaces the old incorrect wasm32 C call ABI with the correct one,
   preserving the old one as wasm32_bindgen_compat for wasm-bindgen
   compatibility.
 - Updates the varargs ABI used by Emscripten and deletes the old one.
 - Removes the obsolete wasm32-experimental-emscripten target.
 - Uses EMCC_CFLAGS on CI to avoid the timeout problems with #63649.

r? @alexcrichton

Author: bors

config.toml.example

@@ -377,6 +377,7 @@
 # but you can also optionally enable the "emscripten" backend for asm.js or
 # make this an empty array (but that probably won't get too far in the
 # bootstrap)
+# FIXME: remove the obsolete emscripten backend option.
 #codegen-backends = ["llvm"]
 
 # This is the name of the directory in which codegen backends will get installed

src/bootstrap/builder.rs

@@ -980,6 +980,7 @@ impl<'a> Builder<'a> {
                 Some("-Wl,-rpath,@loader_path/../lib")
             } else if !target.contains("windows") &&
                       !target.contains("wasm32") &&
+                      !target.contains("emscripten") &&
                       !target.contains("fuchsia") {
                 Some("-Wl,-rpath,$ORIGIN/../lib")
             } else {