Upgrade Emscripten targets to use upstream LLVM backend

- Refactors the Emscripten target spec to share code with other wasm
   targets.
 - Replaces the incorrect wasm32 C call ABI with the old asmjs
   version, which is correct for both wasm32 and JS.
 - Updates the varargs ABI used by Emscripten and deletes the old one.
 - Removes the obsolete wasm32-experimental-emscripten target.
 - Temporarily makes Emscripten targets use panic=abort by default
   because supporting unwinding will require an LLVM patch.

src/test/ui/iterators/iter-sum-overflow-overflow-checks.rs

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