hw/arm_gic: Use NVIC instead of LEGACY_INCLUDED_GIC define

hw/arm_gic: Use NVIC instead of LEGACY_INCLUDED_GIC define

Now all the A profile cores have been switched to use the standalone
sysbus GIC, the only remaining code which #includes arm_gic.c is
the v7M NVIC. The coupling is much closer here so it's not so
easily disentangled. For now, add a comment about how arm_gic.c
is compiled, and assume that the NVIC always includes arm_gic.c
and the non-NVIC GIC is always compiled standalone.

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