[libcxx][CMake] Set library dir when built under runtimes/

This will put libraries into the build root's lib/ directory by default.

Differential Revision: https://reviews.llvm.org/D32736

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@302117 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed