AArch64: add support for ARM64 iOS versions of compiler-rt.

Just a dummy directory and a few sane choices in the Darwin SDK.

rdar://problem/18575597

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@219323 91177308-0d34-0410-b5e6-96231b3b80d8