x86/irq: Unionize PID.PIR for 64bit access w/o casting

Make the PIR field into u64 such that atomic xchg64 can be used without
ugly casting.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240423174114.526704-3-jacob.jun.pan@linux.intel.com