Handle feature=rustbuild

Taken from https://github.com/rust-lang/rust/pull/40347/files

And update rls-span to a version with the rustbuild boilerplate

src/Cargo.lock

@@ -419,7 +419,7 @@ dependencies = [
 [[package]]
 name = "rls-span"
 version = "0.1.0"
-source = "git+https://github.com/nrc/rls-span#e9224b1c52d1d43f9f7b3bb065653c9e18bb532d"
+source = "git+https://github.com/nrc/rls-span#79375240da727c583ce656a6beb034c163313399"
 dependencies = [
  "rustc-serialize 0.3.22 (registry+https://github.com/rust-lang/crates.io-index)",
 ]

src/bootstrap/bin/rustc.rs

@@ -79,6 +79,7 @@ fn main() {
     cmd.args(&args)
         .arg("--cfg")
         .arg(format!("stage{}", stage))
+        .arg("--cfg").arg("rustbuild")
         .env(bootstrap::util::dylib_path_var(),
              env::join_paths(&dylib_path).unwrap());