MIPS64: Initial MIPS64 related change.

With this change Breakpad can be compiled for MIPS64,
but it is not yet functional.

Patch by Gordana Cmiljanovic <Gordana.Cmiljanovic@imgtec.com>
Review URL: https://breakpad.appspot.com/6824002/


git-svn-id: http://google-breakpad.googlecode.com/svn/trunk/src@1446 4c0a9323-5329-0410-9bdc-e9ce6186880e