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/bootstrap/native.rs

@@ -86,6 +86,11 @@ pub fn llvm(build: &Build, target: &str) {
         None => "X86;ARM;AArch64;Mips;PowerPC;SystemZ;JSBackend;MSP430;Sparc;NVPTX;Hexagon",
     };
 
+    let llvm_exp_targets = match build.config.llvm_experimental_targets {
+        Some(ref s) => s,
+        None => "",
+    };
+
     let assertions = if build.config.llvm_assertions {"ON"} else {"OFF"};
 
     cfg.target(target)
@@ -94,6 +99,7 @@ pub fn llvm(build: &Build, target: &str) {
        .profile(profile)
        .define("LLVM_ENABLE_ASSERTIONS", assertions)
        .define("LLVM_TARGETS_TO_BUILD", llvm_targets)
+       .define("LLVM_EXPERIMENTAL_TARGETS_TO_BUILD", llvm_exp_targets)
        .define("LLVM_INCLUDE_EXAMPLES", "OFF")
        .define("LLVM_INCLUDE_TESTS", "OFF")
        .define("LLVM_INCLUDE_DOCS", "OFF")