Revert "[libc++] Take 2: Do not cleverly link against libc++abi just because...

Revert "[libc++] Take 2: Do not cleverly link against libc++abi just because it happens to be there"

r365326 still breaks bots:
http://lab.llvm.org:8011/builders/netbsd-amd64/builds/20712/steps/ninja%20build%20local/logs/stdio
http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-autoconf/builds/39477/steps/test%20tsan%20in%20debug%20compiler-rt%20build/logs/stdio

And probably others

This reverts commit 945b9ec0.

llvm-svn: 365359