Preserve the linkage for objc* intrinsics as clang will set them to weak_external in some cases

Clang uses weak linkage for objc runtime functions when they are not available on the platform.

The intrinsic has this linkage so we just need to pass that on to the runtime call.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@349559 91177308-0d34-0410-b5e6-96231b3b80d8