Add MMX detection when getting registers in Linux.

For CPUs that don't support the MMX instruction set, such pre-Pentium III or industrial x86 embedded PCs, the minidump fails when it tries to retrieve MMX specific registers.

This patch adds MMX detection for that call.

Tested on Ubuntu 12.04 with i686, and on a custom Linux distro on a Vortex86DX microcontroller.

Original review: https://breakpad.appspot.com/455002/
A=aras.vaichas
BUG=495

Review URL: https://breakpad.appspot.com/864002

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1248 4c0a9323-5329-0410-9bdc-e9ce6186880e
Fix Windows release build error due to unreferenced variables.

BUG=544
R=mark@chromium.org

Review URL: https://breakpad.appspot.com/874002

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1249 4c0a9323-5329-0410-9bdc-e9ce6186880e