Fix for libc++abi when using -Wl,--as-needed.

The cmake files for libc++abi and the unwinder weren't linking against
libpthread or an unwind library. If the tests were linked with
-Wl,--as-needed, these libraries wouldn't be linked, causing them to
fail.

Patch contributed by İsmail Dönmez.

llvm-svn: 212958