rustbuild: Compile with the build compiler

This switches the defaults to ensure that everything is built with the build
compiler rather than the host compiler itself (which we're not guaranteed to be
able to run)

Author: alexcrichton

src/bootstrap/build/step.rs

@@ -93,13 +93,13 @@ fn top_level(build: &Build) -> Vec<Step> {
                 continue
             }
             let host = t.target(host);
-            targets.push(host.librustc(stage, host.compiler(stage)));
+            targets.push(host.librustc(stage, t.compiler(stage)));
             for target in build.config.target.iter() {
                 if !build.flags.target.contains(target) {
                     continue
                 }
                 targets.push(host.target(target)
-                                 .libstd(stage, host.compiler(stage)));
+                                 .libstd(stage, t.compiler(stage)));
             }
         }
     }