[lldb] Recognize MachO cpu sub-type 2 as arm64

Make sure we recognize cpu sub-type 2 as arm64. In reality it's arm64e,
but we don't have the triple for that. Without this patch, we fall back
to unknown-apple-macosx- for the default architecture, which breaks
things like running expressions without a target.

Differential revision: https://reviews.llvm.org/D92603