[CMake] Default CMAKE_MACOSX_RPATH to On, this is the default with CMP0042

Summary: Building the sanitizer libraries without rpaths causes all sorts of problems when you try to use them. This simple fix should make it all work.

Reviewers: samsonov, zaks.anna

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D17556

llvm-svn: 261797