Ultimate way to prevent the decrementer from underflowing

[Jakub Jermář]

In kernel/arch/ppc32/src/asm.S, there is:

iret_syscall:

        /* Reset decrementer */

        li r31, 1000
        mtdec r31

This is definitely wrong, especially now when the kernel can program the decrementer with a proper value to achieve its own HZ clock() frequency.

A more systematic approach for preventing the decrementer from underflowing and missing the decrementer exception is needed.