[libcxx][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/D33762

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@307615 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed