Merge Robert Norton's ifdefs to CCall/CReturn allowing various aspects of
its behaviour to be conditionally compiled.  When using hardware assist for
tag/seal/type/permission/range checks, read the faulting instruction's
opcode in order to confirm that the registers checked by CCall are the
registers that we will unseal.  We don't yet have hardware assist for
register clearing, so temporarily impose the presumed cost of those
instructions in nops.  By default, none of these options are compiled into
CHERI kernels.