target-arm: Override do_interrupt for ARMv7-M profile

Enable ARMCPUInfo to specify a custom class_init functions.
Introduce arm_v7m_class_init() and use it for "cortex-m3" model.

Instead of forwarding from arm_cpu_do_interrupt() to do_interrupt_v7m(),
override CPUClass::do_interrupt with arm_v7m_cpu_do_interrupt()
in arm_v7m_class_init().

Acked-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>
