[CMake] Set library dir to be LLVM's intermediate output dir

This matches the behavior of libc++abi and libc++ and ensures that
we get a working toolchain when building libunwind as part of LLVM.

Differential Revision: https://reviews.llvm.org/D34375

llvm-svn: 308380
GitOrigin-RevId: 415813151268f52db8c0a79cdb15b3f9beb78fb6
1 file changed