[CMake] [builtins] Only include the atomic builtins if(APPLE).
[CMake] [builtins] Only include the atomic builtins if(APPLE).

There are lingering issues building the atomic builtins with various versions of gcc. To unblock people we can only include them on Apple platforms where they are more tested.

git-svn-id: c199f293c43da69278bea8e88f92242bf3aa95f7@248386 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt