Fix x doc --stage 0 compiler

Eric figured out the fix to this almost 2 years ago, I just didn't read his comment carefully enough at the timme.
The issue was that fake rustc and fake rustdoc were inconsistent about when they passed `--sysroot` to the real compiler.
Change them to consistently only pass it when `--target` is present.

src/bootstrap/bin/rustdoc.rs

@@ -15,6 +15,10 @@ fn main() {
  let libdir = env::var_os("RUSTDOC_LIBDIR").expect("RUSTDOC_LIBDIR was not set");
  let sysroot = env::var_os("RUSTC_SYSROOT").expect("RUSTC_SYSROOT was not set");
 
+ // Detect whether or not we're a build script depending on whether --target
+ // is passed (a bit janky...)
+ let target = args.windows(2).find(|w| &*w[0] == "--target").and_then(|w| w[1].to_str());
+
  use std::str::FromStr;
 
  let verbose = match env::var("RUSTC_VERBOSE") {
@@ -26,10 +30,18 @@ fn main() {
  dylib_path.insert(0, PathBuf::from(libdir.clone()));
 
  let mut cmd = Command::new(rustdoc);
- cmd.args(&args)
- .arg("--sysroot")
- .arg(&sysroot)
- .env(dylib_path_var(), env::join_paths(&dylib_path).unwrap());
+
+ if target.is_some() {
+ // The stage0 compiler has a special sysroot distinct from what we
+ // actually downloaded, so we just always pass the `--sysroot` option,
+ // unless one is already set.
+ if !args.iter().any(|arg| arg == "--sysroot") {
+ cmd.arg("--sysroot").arg(&sysroot);
+ }
+ }
+
+ cmd.args(&args);
+ cmd.env(dylib_path_var(), env::join_paths(&dylib_path).unwrap());
 
  // Force all crates compiled by this compiler to (a) be unstable and (b)
  // allow the `rustc_private` feature to link to other unstable crates