Fix aarch64 ucontext layout on Android

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

Review URL: https://breakpad.appspot.com/1444002

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1306 4c0a9323-5329-0410-9bdc-e9ce6186880e