Fix makefile build on OSX when ARM targets are not enabled

Summary: When LLVM/Clang is built without ARM support, the ios_kext runtime
library is not built, but without this patch, the Makefile still tries to
copy it. This is a recent regression, because the ios_kext library used
to also be built on x86_64.

Reviewers: beanz

Subscribers: aemerson, cfe-commits, rengolin

Differential Revision: http://reviews.llvm.org/D13421

llvm-svn: 249281