debug/elf: support arm64 relocations

This adds the minimal support for AArch64/arm64 relocations
needed to get cgo to work (when an isomorphic patch is applied
to gccgo) and a test.

This change uses the "AAarch64" name for the architecture rather
than the more widely accepted "arm64" because that's the name that
the relevant docs from ARM such as

   http://infocenter.arm.com/help/topic/com.arm.doc.ihi0056b/IHI0056B_aaelf64.pdf

all use.

Fixes issue 8533.

LGTM=iant
R=golang-codereviews, aram, gobot, iant, minux
CC=golang-codereviews
https://codereview.appspot.com/132000043

Committer: Ian Lance Taylor <iant@golang.org>