AArch64: use __clear_cache under GCCish environments

AArch64 is going to need some kind of cache-invalidation in order to
successfully JIT since it has a weak memory-model. This is provided by
a __clear_cache builtin in libgcc, which acts very much like the
32-bit ARM equivalent (on platforms where it exists).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181129 91177308-0d34-0410-b5e6-96231b3b80d8