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

This reverts r365222, which broke the libc++ build bots.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@365233 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed