hw/arm/virt: Replace TARGET_AARCH64 -> target_aarch64()

Replace the target-specific TARGET_AARCH64 definition
by a call to the generic target_aarch64() helper.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Acked-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20251021210144.58108-4-philmd@linaro.org>