Generate "current" API level.

Support for using this coming in an upcoming patch.

Test: nose2
      readelf to check the following:
      * bsd_signal unversioned before current
      * bsd_signal versioned in current
      * catclose missing before current
      * catclose present and versioned in current
Bug: None
Change-Id: I861862161426d3ec5b530e3156d3a8ae96fed468