UPSTREAM: arm64: Treat handle_arch_irq as a function pointer

handle_arch_irq isn't actually text, it's just a function pointer.
It doesn't need to be stored in the text section and doing so
causes problesm if we ever want to make the kernel text read only.
Declare handle_arch_irq as a proper function pointer stored in
the data section.

Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

(cherry picked from commit fcff588633e848aa728a4437ef96d437299ba03d)
Signed-off-by: Tomasz Figa <tfiga@chromium.org>

Bug: 24475017

Change-Id: I7c842503cd70881fa78bbeb47d98f15bb83f6446
Signed-off-by: Kees Cook <keescook@google.com>