genirq: Allow architectures to override set_handle_irq() fallback

Some architectures want to provide the generic set_handle_irq() API, but
for structural reasons need to provide their own implementation. For
example, arm64 needs to do this to provide uniform set_handle_irq() and
set_handle_fiq() registration functions.

Make this possible by allowing architectures to provide their own
implementation of set_handle_irq when CONFIG_GENERIC_IRQ_MULTI_HANDLER
is not selected.
Signed-off-by: Marc Zyngier <maz@kernel.org>
[Mark: expand commit message]
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Hector Martin <marcan@marcan.st>
Cc: James Morse <james.morse@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20210315115629.57191-2-mark.rutland@arm.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

include/linux/irq.h

@@ -1258,11 +1258,13 @@ int __init set_handle_irq(void (*handle_irq)(struct pt_regs *));
  */
 extern void (*handle_arch_irq)(struct pt_regs *) __ro_after_init;
 #else
+#ifndef set_handle_irq
 #define set_handle_irq(handle_irq)		\
 	do {					\
 		(void)handle_irq;		\
 		WARN_ON(1);			\
 	} while (0)
 #endif
+#endif
 
 #endif /* _LINUX_IRQ_H */