style: consistently attach return type

include/api/debug.h

include/api/syscall.h

include/api/types.h

include/arch/arm/arch/32/mode/fastpath/fastpath.h

include/arch/arm/arch/32/mode/kernel/thread.h

include/arch/arm/arch/32/mode/machine/debug.h

include/arch/arm/arch/32/mode/machine/fpu.h

include/arch/arm/arch/32/mode/machine/hardware.h

include/arch/arm/arch/32/mode/object/structures.h

include/arch/arm/arch/32/mode/smp/smp.h

include/arch/arm/arch/64/mode/fastpath/fastpath.h

include/arch/arm/arch/64/mode/kernel/thread.h

include/arch/arm/arch/64/mode/machine/hardware.h

include/arch/arm/arch/64/mode/object/structures.h

include/arch/arm/arch/benchmark.h

include/arch/arm/arch/machine/debug.h

include/arch/arm/arch/machine/generic_timer.h

include/arch/arm/arch/machine/gic_pl390.h

include/arch/arm/arch/model/smp.h

include/arch/arm/arch/object/interrupt.h