Copy cheri.c as a starting point for cheri_exception.c.
Move most exception-related functionality from cheri.c to cheri_exception.c.
Copy cheri.c as a starting point for cheri_signal.c.
Break out signal-delivery-related contents of cheri.c to cheri_signal.c.
Compile new cheri_exception.c and cheri_signal.c, with suitable header
extensions for now-global variables.
Copy cheri.c as a starting point for cheri_debug.c.
Move DDB-related functions from cheri.c to cheri_debug.c.
Relocate last bit off DDB-related debugging code to cheri_debug.c.  Trim
some now-unneeded includes of opt_ddb.h.