Add unwinder linking to STL setup.

Previously we were bundling this into libc++ and having each binary
resolve its symbols from there. This means that the unwinder that gets
used for a given binary is sensitive to the order in which its
libraries are linked. Making this a static library dependency of each
binary means the right unwinder will always be used.

Bug: 18471342
Bug: 19958712
Change-Id: I782d212784dbf85355b5d9146d7460d675259a0e