cpu_check_irqs
- handle SOFTINT register TICK and STICK timer bits
- only check interrupt levels greater than PIL value
- handle preemption by higher level traps

cpu_exec
- handle CPU_INTERRUPT_HARD only if interrupts are enabled
- PIL 15 is not special level on sparcv9

Signed-off-by: Igor V. Kovalenko <igor.v.kovalenko@gmail.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>