Compile arm_gic.c as a standalone C file to produce a self contained
sysbus GIC device. Support the legacy usage by #include of the .c file
by making those users #define LEGACY_INCLUDED_GIC, so we can convert
them one by one.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NEvgeny Voevodin <e.voevodin@samsung.com>