Add x64 version of getcontext.

Assembly code is derived in part from code in libunwind.
Code tested on desktop linux (Android testing pending emulation
support).

BUG=346626
R=dannyb@google.com, thestig@chromium.org

Review URL: https://breakpad.appspot.com/1454002

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1311 4c0a9323-5329-0410-9bdc-e9ce6186880e