[CMake] Use LIBCXXABI_LIBDIR_SUFFIX in libc++abi build

This was changed unintentionally in r335809.

Differential Revision: https://reviews.llvm.org/D49765

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@337937 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed