cpu-hotplug: Provide prototypes for arch CPU registration

Provide common prototypes for arch_register_cpu() and
arch_unregister_cpu(). These are called by acpi_processor.c, with weak
versions, so the prototype for this is already set. It is generally not
necessary for function prototypes to be conditional on preprocessor macros.

Some architectures (e.g. Loongarch) are missing the prototype for this, and
rather than add it to Loongarch's asm/cpu.h, do the job once for everyone.

Since this covers everyone, remove the now unnecessary prototypes in
asm/cpu.h, and therefore remove the 'static' from one of ia64's
arch_register_cpu() definitions.

[ tglx: Bring back the ia64 part and remove the ACPI prototypes ]

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/E1qkoRr-0088Q8-Da@rmk-PC.armlinux.org.uk