hw/intc/armv7m_nvic: Only show ID register values for Main Extension CPUs

M-profile CPUs only implement the ID registers as guest-visible if
the CPU implements the Main Extension (all our current CPUs except
the Cortex-M0 do).

Currently we handle this by having the Cortex-M0 leave the ID
register values in the ARMCPU struct as zero, but this conflicts with
our design decision to make QEMU behaviour be keyed off ID register
fields wherever possible.

Explicitly code the ID registers in the NVIC to return 0 if the Main
Extension is not implemented, so we can make the M0 model set the
ARMCPU struct fields to obtain the correct behaviour without those
values becoming guest-visible.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200910173855.4068-4-peter.maydell@linaro.org

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