[libunwind][CMake] Add install path variable to allow overriding the destination

This is going to be used by the runtime build in the multi-target
setup to allow using different install prefix for each target.

Differential Revision: https://reviews.llvm.org/D33760

llvm-svn: 307606
GitOrigin-RevId: 8bb0f6ec88a3bc205a440bb9ac59e5a8d8daae31
2 files changed