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_llvm/build.rs

@@ -93,7 +93,7 @@ fn main() {
 
     let mut optional_components =
         vec!["x86", "arm", "aarch64", "mips", "powerpc", "pnacl",
-             "systemz", "jsbackend", "msp430", "sparc", "nvptx"];
+             "systemz", "jsbackend", "webassembly", "msp430", "sparc", "nvptx"];
 
     let mut version_cmd = Command::new(&llvm_config);
     version_cmd.arg("--version");

src/librustc_llvm/lib.rs

@@ -389,6 +389,11 @@ pub fn initialize_available_targets() {
                  LLVMInitializeHexagonTargetMC,
                  LLVMInitializeHexagonAsmPrinter,
                  LLVMInitializeHexagonAsmParser);
+    init_target!(llvm_component = "webassembly",
+                 LLVMInitializeWebAssemblyTargetInfo,
+                 LLVMInitializeWebAssemblyTarget,
+                 LLVMInitializeWebAssemblyTargetMC,
+                 LLVMInitializeWebAssemblyAsmPrinter);
 }
 
 pub fn last_error() -> Option<String> {