Allow building without LTO

This is intended for use with try builders to speed up presubmits.

Change-Id: Ib8c6a6de07d9e41c30926071fbf3dea4de99942e
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/5640
Commit-Queue: Christopher Fujino <fujino@google.com>
Commit-Queue: Dan Field <dnfield@google.com>
Reviewed-by: Godofredo Contreras <godofredoc@google.com>