Check CPU features in the Simulator and Disassembler.

This is implemented using a new decoder visitor, CPUFeaturesAuditor. This
visitor holds a set of features required by the last-decoded instruction, a set
of features seen since construction (or 'ResetSeenFeatures()'), and a set of
features that are considered to be available. This visitor can be used by
itself, but its main purpose is to implement Simulator and Disassembler support
for CPUFeatures:

- The Simulator creates and attaches this visitor automatically, much like it
  does for the disassembly trace, so existing simulation use-cases require no
  special user interaction. The Simulator checks that each encountered
  instruction only uses available features.

  The Simulator also supports dynamic configuration of features, in a manner
  similar to the trace controls.

- The Disassembler does not directly interact with the CPUFeaturesAuditor, but
  the PrintDisassembler can be configured to annotate each instruction with
  required features that are missing from GetAvailableFeatures().

  The PrintDisassembler is used to implement Simulator trace, so a simulation
  failure due to missing feature support will be immediately preceeded by a
  corresponding PrintDisassembler annotation (when trace is enabled).

  No existing test uses the PrintDisassembler in this way, so this patch also
  adds 'TRACE_' tests to check that the features are appropriately formatted.

Change-Id: I3dc0e22f0a5bd3287ba1870539e018ff4a768a4b