Add target to use LLVM wasm backend

The new target is wasm32-experimental-emscripten. Adds a new
configuration option to opt in to building experimental LLVM backends
such as the WebAssembly backend. The target name was chosen to be
similar to the existing wasm32-unknown-emscripten target so that the
build and tests would work with minimal other code changes. When/if the
new target replaces the old target, simply renaming it should just work.

src/librustc_back/target/mod.rs

@@ -214,6 +214,7 @@ supported_targets! {
     ("le32-unknown-nacl", le32_unknown_nacl),
     ("asmjs-unknown-emscripten", asmjs_unknown_emscripten),
     ("wasm32-unknown-emscripten", wasm32_unknown_emscripten),
+    ("wasm32-experimental-emscripten", wasm32_experimental_emscripten),
 
     ("thumbv6m-none-eabi", thumbv6m_none_eabi),
     ("thumbv7m-none-eabi", thumbv7m_none_eabi),

src/librustc_back/target/wasm32_experimental_emscripten.rs

@@ -0,0 +1,53 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use LinkerFlavor;
+use super::{LinkArgs, Target, TargetOptions};
+use super::emscripten_base::{cmd};
+
+pub fn target() -> Result<Target, String> {
+    let mut post_link_args = LinkArgs::new();
+    post_link_args.insert(LinkerFlavor::Em,
+                          vec!["-s".to_string(),
+                               "WASM=1".to_string(),
+                               "-s".to_string(),
+                               "ERROR_ON_UNDEFINED_SYMBOLS=1".to_string()]);
+
+    let opts = TargetOptions {
+        linker: cmd("emcc"),
+        ar: cmd("emar"),
+
+        dynamic_linking: false,
+        executables: true,
+        // Today emcc emits two files - a .js file to bootstrap and
+        // possibly interpret the wasm, and a .wasm file
+        exe_suffix: ".js".to_string(),
+        linker_is_gnu: true,
+        allow_asm: false,
+        obj_is_bitcode: true,
+        is_like_emscripten: true,
+        max_atomic_width: Some(32),
+        post_link_args: post_link_args,
+        target_family: Some("unix".to_string()),
+        .. Default::default()
+    };
+    Ok(Target {
+        llvm_target: "wasm32-unknown-unknown".to_string(),
+        target_endian: "little".to_string(),
+        target_pointer_width: "32".to_string(),
+        target_os: "emscripten".to_string(),
+        target_env: "".to_string(),
+        target_vendor: "unknown".to_string(),
+        data_layout: "e-m:e-p:32:32-i64:64-n32:64-S128".to_string(),
+        arch: "wasm32".to_string(),
+        linker_flavor: LinkerFlavor::Em,
+        options: opts,
+    })
+}