Do not track the sysroot.

As suggested by @alexcrichton, the sysroot only loads libraries that
are themselves tracked.

src/librustc/session/config.rs

@@ -396,7 +396,6 @@ top_level_options!(
         search_paths: Vec<SearchPath> [UNTRACKED],
         libs: Vec<(String, Option<String>, Option<cstore::NativeLibraryKind>)> [TRACKED],
         maybe_sysroot: Option<PathBuf> [UNTRACKED],
-        maybe_sysroot_remapped: Option<PathBuf> [TRACKED],
 
         target_triple: TargetTriple [TRACKED],
 
@@ -611,7 +610,6 @@ impl Default for Options {
             output_types: OutputTypes(BTreeMap::new()),
             search_paths: vec![],
             maybe_sysroot: None,
-            maybe_sysroot_remapped: None,
             target_triple: TargetTriple::from_triple(host_triple()),
             test: false,
             incremental: None,
@@ -2455,7 +2453,7 @@ pub fn build_session_options_and_crate_config(
 
     let crate_name = matches.opt_str("crate-name");
 
-    let remap_path_prefix: Vec<(PathBuf, PathBuf)> = matches
+    let remap_path_prefix = matches
         .opt_strs("remap-path-prefix")
         .into_iter()
         .map(|remap| {
@@ -2472,10 +2470,6 @@ pub fn build_session_options_and_crate_config(
         })
         .collect();
 
-    let sysroot_remapped_opt = sysroot_opt
-        .clone()
-        .map(|sysroot| FilePathMapping::new(remap_path_prefix.clone()).map_prefix(sysroot).0);
-
     (
         Options {
             crate_types,
@@ -2487,7 +2481,6 @@ pub fn build_session_options_and_crate_config(
             output_types: OutputTypes(output_types),
             search_paths,
             maybe_sysroot: sysroot_opt,
-            maybe_sysroot_remapped: sysroot_remapped_opt,
             target_triple,
             test,
             incremental,