Revert "[libc++] Take 3: Do not cleverly link against libc++abi just because...

Revert "[libc++] Take 3: Do not cleverly link against libc++abi just because it happens to be there"

This also reverts "[libc++] Remove temporary hack for D63883".
Clearly, I don't understand how the Linux build bots are configured.

Differential Revision: https://reviews.llvm.org/D63883

llvm-svn: 368238