ACPICA: Linux-specific header: enable "aarch64" 64-bit build.

Add support for the __aarch64__ define for 64-bit builds.

Signed-off-by: Naresh Bhat <naresh.bhat@linaro.org>
Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Reviewed-by: Len Brown <len.brown@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>