[CMake] Fix statically linking in libcxxabi if built separately

In this case, CMake doesn't know about the c++abi target within the
same CMake run.

This reverts this aspect back to how it was before SVN r357811.

Differential Revision: https://reviews.llvm.org/D60448

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@358009 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed