[CMake] Tell libc++ that we're using compiler-rt on Apple platforms

Reviewers: beanz, arphaman, EricWF

Subscribers: dberris, mgorny, jkorous, dexonsmith, jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58921

llvm-svn: 355927