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Introduce "just interpret" chainable pseudo-translation.

This is the first step towards enabling translation & self-cosim stress modes.
When trace selection begins, the trace head address is pinned and
remains in a limbo state until the translation is complete.  Previously,
if the trace selected aborted for any reason, the trace head would remain
forever in limbo.  This was not a correctness problem, but caused some
small performance anomolies and made life more difficult for self-cosimulation
mode.

This CL introduces a pseudo-translation that simply routes control to
the interpreter.  When we detect that a trace selection attempt has
failed, the trace head is associated with this fully-chainable
pseudo-translation.  This also has the benefit for self-cosimulation that
we are guaranteed forward progress.