Respect LIBCXXABI_LIBDIR_SUFFIX before an install

Prior to this patch, setting LIBCXXABI_LIBDIR_SUFFIX would confuse the
check-libcxxabi target. libc++abi.* would get output to lib instead of
lib${LIBCXXABI_LIBDIR_SUFFIX}, but the tests would look in the suffixed
directory.

Now, we match what libcxx does, and set the CMAKE_*_OUTPUT_DIRECTORY to the
LIBRARY_DIR.

http://reviews.llvm.org/D17410

llvm-svn: 266611