travis: Add a distcheck target

This commit adds a new entry to the Travis matrix which performs a "distcheck",
which basically means that we create a tarball, extract that tarball, and then
build/test inside there. This ensures that the tarballs we produce are actually
able to be built/tested!

Along the way this also updates the rustbuild distcheck definition to propagate
the configure args from the top-level invocation.

Closes #38691

src/bootstrap/check.rs

@@ -554,6 +554,7 @@ pub fn distcheck(build: &Build) {
        .current_dir(&dir);
     build.run(&mut cmd);
     build.run(Command::new("./configure")
+                     .args(&build.config.configure_args)
                      .current_dir(&dir));
     build.run(Command::new(build_helper::make(&build.config.build))
                      .arg("check")