Build rustdoc with the stageN compiler in N >= 2.

This permits proc macro crates to correctly work with rustdoc.

src/bootstrap/tool.rs

@@ -260,6 +260,10 @@ impl Step for Rustdoc {
         let target = target_compiler.host;
         let build_compiler = if target_compiler.stage == 0 {
             builder.compiler(0, builder.build.build)
+        } else if target_compiler.stage >= 2 {
+            // Past stage 2, we consider the compiler to be ABI-compatible and hence capable of
+            // building rustdoc itself.
+            target_compiler
         } else {
             // Similar to `compile::Assemble`, build with the previous stage's compiler. Otherwise
             // we'd have stageN/bin/rustc and stageN/bin/rustdoc be effectively different stage