Move X8632-specific Assembler stuff to Machine Traits.

As part of the refactoring moves the MachineTraits<TargetX8632> to a separate header.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4077
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/1216033004.