Stop libc from cross-referencing unwind symbols

  This makes unwind symbols 'protected',
  which should prevent them from relocating
  against libc++.so/libcutls.so.

  This is temporary file and it is going
  to be removed once libc.so stops exporting
  them.

Bug: http://b/19958712
Change-Id: I96a765afe47e68d2e2ceb288870e63a25ca52081