Bazel on Mac requires some alterations to the generated build files. This change updates generate_build_files.py to emit suitable Bazel files. This will require some tweaks to projects that build with Bazel. Change-Id: I3d68ec754b8abaa41a348f86c32434477f2c5e1c Reviewed-on: https://boringssl-review.googlesource.com/6146 Reviewed-by: Adam Langley <agl@google.com>