[cleanup] Refactor parts of builtins-number-gen.cc

Bug: v8:6474

Change-Id: I24d067ac77c2cb557d0b3f6e2c8c512761c160f1
Reviewed-on: https://chromium-review.googlesource.com/531384
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45944}

tools/dev/gm.py

@@ -244,7 +244,8 @@ class Config(object):
     return_code, output = _CallWithOutput("ninja -C %s %s %s" %
                                           (path, BUILD_OPTS, targets))
     if return_code != 0 and "FAILED: gen/snapshot.cc" in output:
-      print("Detected mksnapshot failure, re-running in GDB...")
+      _Notify("V8 build requires your attention",
+              "Detected mksnapshot failure, re-running in GDB...")
       _Call("gdb -args %(path)s/mksnapshot "
             "--startup_src %(path)s/gen/snapshot.cc "
             "--random-seed 314159265 "