Fill in CPU info in mini-dump for Arm64.

Adds Arm64 varients of CPUFillThreadInfo and CPUFillFromUContext and
WriteCPUInformation for the Linux/Android client.

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

Review URL: https://breakpad.appspot.com/1464002

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1310 4c0a9323-5329-0410-9bdc-e9ce6186880e