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

arch/x86/include/asm/posted_intr.h

@@ -9,7 +9,10 @@
 
 /* Posted-Interrupt Descriptor */
 struct pi_desc {
-	u32 pir[8];     /* Posted interrupt requested */
+	union {
+		u32 pir[8];     /* Posted interrupt requested */
+		u64 pir64[4];
+	};
 	union {
 		struct {
 				/* bit 256 - Outstanding Notification */