sparc/shared/irq_asm.S: Get pointer to self per-CPU control

_CPU_Context_switch_to_first_task_smp() branches to
done_flushing which requires o3 to be initalized with
"self per-CPU control", this adds initialization of
o3. This problem only affects SMP, see commit
f8ad6c6f7f8b591e were usage of o3 was intruduced.