[CMake] Allow overriding lib dir suffix independently from LLVM

This matches the options already supported by libc++ and libc++abi.

Differential Revision: https://reviews.llvm.org/D36383

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@310327 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed