Add empty placeholder git/gen files.

Projects that depend on TensorFlow as a Bazel external dependency will
almost certainly not be modifying any of the TensorFlow source and therefore
don't need these git symlinked files (which exist for Bazel to keep track
of changes to HEAD).

Adding empty files for the files generated by configure so that TensorFlow
can be built without running configure for these projects.

These placeholders will get overridden for TF checkouts the first time you
run configure.py.

PiperOrigin-RevId: 177184829