Add Arm64 support to dumpsyms.

Adds Arm64 support to dumpsyms, enabling support for EM_AARCH64 elf type and
arm64 registers in DwarfCFIToModule.

BUG=367367,335641,354405
R=mark@chromium.org

Review URL: https://breakpad.appspot.com/1654002

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1322 4c0a9323-5329-0410-9bdc-e9ce6186880e