Move all platform/default build targets from tf/core/platform/default/build_refactor.bzl into tf/core/platform/default/BUILD.

Subsequent changes will change tensorflow/core/platform/BUILD to use these targets, and remove build_refactor.bzl.
This refactoring is part of the Tensorflow Build Improvements RFC described here: tensorflow/community#179

PiperOrigin-RevId: 284885829
Change-Id: I80b0722f11d89b4cb0bef628beedd24d882ac1b4

Author: bmzhao