Label more build steps.

Currently the output of a command like `./x.py build --stage 0 library/std` is this:
```
Updating only changed submodules
Submodules updated in 0.02 seconds
extracting [...]
   Compiling [...]
    Finished dev [unoptimized] target(s) in 17.53s
Building stage0 std artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
   Compling [...]
    Finished release [optimized + debuginfo] target(s) in 21.99s
Copying stage0 std from stage0 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / x86_64-unknown-linux-gnu)
Build completed successfully in 0:00:51
```
I find the part before the "Building stage0 std artifacts" a bit confusing.

After this commit, it looks like this:
```
Updating only changed submodules
  Submodules updated in 0.02 seconds
extracting [...]
Building rustbuild
   Compiling [...]
    Finished dev [unoptimized] target(s) in 17.53s
Building stage0 std artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
   Compling [...]
    Finished release [optimized + debuginfo] target(s) in 21.99s
Copying stage0 std from stage0 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / x86_64-unknown-linux-gnu)
Build completed successfully in 0:00:51
```
The "Building rustbuild" label makes it clear what the first cargo build
invocation is for. The indentation of the "Submodules updated" line
indicates it is a sub-step of a parent task.

Author: nnethercote

src/bootstrap/bootstrap.py

@@ -974,6 +974,7 @@ def bootstrap_binary(self):
 
     def build_bootstrap(self):
         """Build bootstrap"""
+        print("Building rustbuild")
         build_dir = os.path.join(self.build_dir, "bootstrap")
         if self.clean and os.path.exists(build_dir):
             shutil.rmtree(build_dir)
@@ -1133,7 +1134,7 @@ def update_submodules(self):
             recorded_submodules[data[3]] = data[2]
         for module in filtered_submodules:
             self.update_submodule(module[0], module[1], recorded_submodules)
-        print("Submodules updated in %.2f seconds" % (time() - start_time))
+        print("  Submodules updated in %.2f seconds" % (time() - start_time))
 
     def set_dist_environment(self, url):
         """Set download URL for normal environment"""