gicv3_set_gicv3state() is used by arm_gicv3_common.c in
arm_gicv3_common_realize(). Since we want to restrict
arm_gicv3_cpuif.c to TCG, extract gicv3_set_gicv3state()
to a new file. Add this file to the meson 'specific'
source set, since it needs access to "cpu.h".

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20211115223619.2599282-2-philmd@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>