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