AArch64: teach Clang about __clear_cache intrinsic

libgcc provides a __clear_cache intrinsic on AArch64, much like it
does on 32-bit ARM.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181111 91177308-0d34-0410-b5e6-96231b3b80d8