rustc: Prepare the atomics feature for wasm

This commit adds a few changes for atomic instructions on the
`wasm32-unknown-unknown` target. Atomic instructions are not yet stable in
WebAssembly itself but there are multiple implementations and LLVM has support
for the proposed instruction set, so let's work on exposing it!

Here there are a few inclusions:

* The `atomics` feature was whitelisted for LLVM, allowing code in Rust to
  enable/disable/gate on this.

* The `singlethread` option is turned off for wasm when the `atomics` feature is
  enabled. This means that by default wasm won't be lowering with atomics, but
  when atomics are enabled globally we'll turn off single-threaded mode to
  actually codegen atomics. This probably isn't what we'll want in the long term
  but for now it should work.

* Finally the maximum atomic width is increased to 64 to reflect the current
  wasm spec.

src/librustc_codegen_llvm/back/write.rs

@@ -171,13 +171,22 @@ pub fn target_machine_factory(sess: &Session, find_features: bool)
         None => llvm::CodeModel::None,
     };
 
-    let singlethread = sess.target.target.options.singlethread;
+    let features = attributes::llvm_target_features(sess).collect::<Vec<_>>();
+    let mut singlethread = sess.target.target.options.singlethread;
+
+    // On the wasm target once the `atomics` feature is enabled that means that
+    // we're no longer single-threaded, or otherwise we don't want LLVM to
+    // lower atomic operations to single-threaded operations.
+    if singlethread &&
+        sess.target.target.llvm_target.contains("wasm32") &&
+        features.iter().any(|s| *s == "+atomics")
+    {
+        singlethread = false;
+    }
 
     let triple = SmallCStr::new(&sess.target.target.llvm_target);
     let cpu = SmallCStr::new(llvm_util::target_cpu(sess));
-    let features = attributes::llvm_target_features(sess)
-        .collect::<Vec<_>>()
-        .join(",");
+    let features = features.join(",");
     let features = CString::new(features).unwrap();
     let is_pie_binary = !find_features && is_pie_binary(sess);
     let trap_unreachable = sess.target.target.options.trap_unreachable;

src/librustc_codegen_llvm/llvm_util.rs

@@ -173,6 +173,7 @@ const MIPS_WHITELIST: &[(&str, Option<&str>)] = &[
 
 const WASM_WHITELIST: &[(&str, Option<&str>)] = &[
     ("simd128", Some("wasm_target_feature")),
+    ("atomics", Some("wasm_target_feature")),
 ];
 
 /// When rustdoc is running, provide a list of all known features so that all their respective

src/librustc_target/spec/wasm32_unknown_unknown.rs

@@ -36,8 +36,7 @@ pub fn target() -> Result<Target, String> {
         dll_suffix: ".wasm".to_string(),
         linker_is_gnu: false,
 
-        // A bit of a lie, but "eh"
-        max_atomic_width: Some(32),
+        max_atomic_width: Some(64),
 
         // Unwinding doesn't work right now, so the whole target unconditionally
         // defaults to panic=abort. Note that this is guaranteed to change in