Add DWARF register mappings for ARM64, so that that arch will become
usable with "-dwarflocationlists". [NB: I've plugged in a set of
numbers from the doc, but this will require additional manual testing.]

Change-Id: Id9aa63857bc8b4f5c825f49274101cf372e9e856
Reviewed-on: https://go-review.googlesource.com/82515Reviewed-by: Heschi Kreinick <heschi@google.com>