hw/armv7m_nvic: Make the NVIC a freestanding class

Rearrange the GIC and NVIC so both are straightforward
subclasses of a common class, rather than having the NVIC
source file textually include arm_gic.c.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

hw/arm/Makefile.objs

@@ -6,7 +6,7 @@ obj-y += cadence_uart.o
 obj-y += cadence_ttc.o
 obj-y += cadence_gem.o
 obj-y += xilinx_zynq.o zynq_slcr.o
-obj-y += arm_gic.o
+obj-y += arm_gic.o arm_gic_common.o
 obj-y += realview_gic.o realview.o arm_sysctl.o arm11mpcore.o a9mpcore.o
 obj-y += exynos4210_gic.o exynos4210_combiner.o exynos4210.o
 obj-y += exynos4_boards.o exynos4210_uart.o exynos4210_pwm.o