minidump_stackwalk should display r12 in ARM stack traces
A=torne@google.com, R=ted, http://code.google.com/p/google-breakpad/issues/detail?id=456

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1151 4c0a9323-5329-0410-9bdc-e9ce6186880e