Enable --cfg=parallel_compiler in rustdoc.

Author: m-ou-se

src/bootstrap/tool.rs

@@ -551,7 +551,7 @@ impl Step for Rustdoc {
             features.push("jemalloc".to_string());
         }
 
-        let cargo = prepare_tool_cargo(
+        let mut cargo = prepare_tool_cargo(
             builder,
             build_compiler,
             Mode::ToolRustc,
@@ -562,6 +562,10 @@ impl Step for Rustdoc {
             features.as_slice(),
         );
 
+        if builder.config.rustc_parallel {
+            cargo.rustflag("--cfg=parallel_compiler");
+        }
+
         builder.info(&format!(
             "Building rustdoc for stage{} ({})",
             target_compiler.stage, target_compiler.host