[cmake] [libcxx] Call llvm_setup_rpath() when adding shared libraries.
[cmake] [libcxx] Call llvm_setup_rpath() when adding shared libraries.

Clang and llvm already use llvm_setup_rpath(), so this change will
help standarize rpath usage across all projects.

Differential Revision: https://reviews.llvm.org/D42459

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@323453 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx