cris/kgdb: Use #ifdef PROCESS_SUPPORT where needed

arch/cris/arch-v10/kernel/kgdb.c:354:12: warning: 'current_thread_c' defined but not used [-Wunused-variable]
arch/cris/arch-v10/kernel/kgdb.c:355:12: warning: 'current_thread_g' defined but not used [-Wunused-variable]
arch/cris/arch-v10/kernel/kgdb.c:359:18: warning: 'reg_g' defined but not used [-Wunused-variable]
arch/cris/arch-v10/kernel/kgdb.c:622:1: warning: 'copy_registers' defined but not used [-Wunused-function]

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>