Make x test --stage 2 compiler/rustc_XXX faster to run

Previously, bootstrap unconditionally rebuilt the stage 2 compiler,
even if it had previously built stage 1. This changes it to reuse stage 1 if possible.

Author: jyn514

src/bootstrap/test.rs

@@ -1899,7 +1899,8 @@ impl Step for CrateLibrustc {
 
     fn make_run(run: RunConfig<'_>) {
         let builder = run.builder;
-        let compiler = builder.compiler(builder.top_stage, run.build_triple());
+        let host = run.build_triple();
+        let compiler = builder.compiler_for(builder.top_stage, host, host);
         let krate = builder.crate_paths[&run.path];
         let test_kind = builder.kind.into();
 
@@ -1936,7 +1937,8 @@ impl Step for Crate {
 
     fn make_run(run: RunConfig<'_>) {
         let builder = run.builder;
-        let compiler = builder.compiler(builder.top_stage, run.build_triple());
+        let host = run.build_triple();
+        let compiler = builder.compiler_for(builder.top_stage, host, host);
         let test_kind = builder.kind.into();
         let krate = builder.crate_paths[&run.path];