[AArch64] Add 'apple-latest' CPU alias

The 'apple-latest' alias is supposed to provide a CPU that contains the
latest Apple processor model supported by LLVM.

This is supposed to be used by tools like lldb to provide a target that
supports most of the CPU features.

For now, this is mapped to Cyclone.

Differential Revision: https://reviews.llvm.org/D56384

llvm-svn: 352412