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.

src/test/ui/issues/issue-14875.rs

@@ -1,5 +1,5 @@
 // run-pass
-// ignore-wasm32-bare always compiled as panic=abort right now
+// ignore-emscripten compiled with panic=abort by default
 
 // Check that values are not leaked when a dtor panics (#14875)
 

src/test/ui/issues/issue-23477.rs

@@ -1,4 +1,5 @@
 // build-pass
+// ignore-asmjs wasm2js does not support source maps yet
 // compile-flags: -g
 
 pub struct Dst {

src/test/ui/issues/issue-24687-embed-debuginfo/main.rs

@@ -1,6 +1,7 @@
 // run-pass
 // aux-build:issue-24687-lib.rs
 // compile-flags:-g
+// ignore-asmjs wasm2js does not support source maps yet
 
 extern crate issue_24687_lib as d;
 

src/test/ui/issues/issue-24945-repeat-dash-opts.rs

@@ -3,6 +3,7 @@
 // as options to the compiler.
 
 // compile-flags:-g -g -O -O
+// ignore-asmjs wasm2js does not support source maps yet
 
 fn main() {
     assert_eq!(1, 1);

src/test/ui/issues/issue-26484.rs

@@ -1,5 +1,6 @@
 // run-pass
 // compile-flags:-g
+// ignore-asmjs wasm2js does not support source maps yet
 
 fn helper<F: FnOnce(usize) -> bool>(_f: F) {
     print!("");

src/test/ui/issues/issue-29948.rs

@@ -1,5 +1,5 @@
 // run-pass
-// ignore-wasm32-bare compiled with panic=abort by default
+// ignore-emscripten compiled with panic=abort by default
 
 use std::panic;
 

src/test/ui/issues/issue-33096.rs

@@ -1,5 +1,6 @@
 // run-pass
 // compile-flags: -g
+// ignore-asmjs wasm2js does not support source maps yet
 
 use std::ops::Deref;
 

src/test/ui/issues/issue-33992.rs

@@ -1,7 +1,7 @@
 // run-pass
 // ignore-windows
 // ignore-macos
-// ignore-wasm32-bare common linkage not implemented right now
+// ignore-emscripten common linkage not implemented right now
 
 #![feature(linkage)]
 

src/test/ui/issues/issue-34569.rs

@@ -1,5 +1,6 @@
 // run-pass
 // compile-flags:-g
+// ignore-asmjs wasm2js does not support source maps yet
 
 // In this test we just want to make sure that the code below does not lead to
 // a debuginfo verification assertion during compilation. This was caused by the

src/test/ui/issues/issue-36856.rs

@@ -2,6 +2,7 @@
 // Regression test for #36856.
 
 // compile-flags:-g
+// ignore-asmjs wasm2js does not support source maps yet
 
 fn g() -> bool {
     false

src/test/ui/issues/issue-42210.rs

@@ -2,6 +2,7 @@
 // Regression test for #42210.
 
 // compile-flags: -g
+// ignore-asmjs wasm2js does not support source maps yet
 
 trait Foo {
     fn foo() { }

src/test/ui/issues/issue-43853.rs

@@ -1,5 +1,5 @@
 // run-pass
-// ignore-wasm32-bare compiled with panic=abort by default
+// ignore-emscripten compiled with panic=abort by default
 
 use std::panic;
 

src/test/ui/issues/issue-45731.rs

@@ -1,6 +1,7 @@
 // run-pass
 #![allow(unused_variables)]
 // compile-flags:--test -g
+// ignore-asmjs wasm2js does not support source maps yet
 
 #[cfg(target_os = "macos")]
 #[test]

src/test/ui/issues/issue-46519.rs

@@ -1,6 +1,8 @@
 // run-pass
 // compile-flags:--test -O
 
+// ignore-emscripten compiled with panic=abort by default
+
 #[test]
 #[should_panic(expected = "creating inhabited type")]
 fn test() {

src/test/ui/issues/issue-48508.rs

@@ -8,6 +8,7 @@
 
 // compile-flags:-g
 // ignore-pretty issue #37195
+// ignore-asmjs wasm2js does not support source maps yet
 
 #![feature(non_ascii_idents)]
 

src/test/ui/issues/issue-49579.rs

@@ -1,5 +1,4 @@
 // build-pass (FIXME(62277): could be check-pass?)
-// ignore-emscripten no i128 support
 
 fn fibs(n: u32) -> impl Iterator<Item=u128> {
     (0 .. n)

src/test/ui/issues/issue-58463.rs

@@ -1,5 +1,7 @@
 // run-pass
 // compile-flags:-C debuginfo=2
+// ignore-asmjs wasm2js does not support source maps yet
+
 fn foo() -> impl Copy {
     foo
 }