locore calls to go via a locore-entry jumptable.
Declare r2000- and r4000-specific exception-handler functions, to which
trap() and interrupt() dispatch exceptions. Initialize r2000- and r4000-
specific exception-handler vectors, when CPU_R4000 and CPU_R2000 are
defined.
Update the stack-traceback code (partially) to understand and print
the new low-level exception-handler code, via which machine exception-vectors
send exceptions to call trap() or interrupt(). This needs more work.
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