Make various instances of C-language CHERI register use in the kernel
conform to the new CHERI micro-ABI by using $c11 and $c12 as temporary
registers.

Because these are properly maintained over both voluntary and
involuntary kernel context switches, remove code enabling and disabling
interrupts around C-language use of CHERI registers -- e.g., during
memory copy, context setup, debugging, etc.
Remove stale KDC definition, possibly dating from CHERI compositor work?