Fill in CPU info in mini-dump for Arm64.

Adds Arm64 varients of CPUFillThreadInfo and CPUFillFromUContext and
WriteCPUInformation for the Linux/Android client.

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

Review URL: https://breakpad.appspot.com/1464002

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1310 4c0a9323-5329-0410-9bdc-e9ce6186880e
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