Generate minidumps for 64-bit ARM apps on iOS.

Adds an ARM64-specific definition of MDRawContext and support for writing out a
minidump when running on ARM64. Additionally, extends the iOS minidump generator
for NSExceptions to work on ARM64 as well as ARM.

Patch by Colin Blundell <blundell@chromium.org>

BUG=542

Review URL: https://breakpad.appspot.com/664002/


git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1235 4c0a9323-5329-0410-9bdc-e9ce6186880e