Don't export new/delete from libc shared libraries.

This used to be handled by -fvisibility=hidden on libc_cxa, but that
was broken by the resolution of https://llvm.org/PR22419 (introduced
to Android in today's clang update).

Now we just use a version script that prevents these from being
re-exported from our shared libraries.

Change-Id: Ib290e1d0d7426e09ad17a91178162fff6dbdcfa9