tcg: Define guest_default_memory_order in TCGCPUOps

tcg: Define guest_default_memory_order in TCGCPUOps

Add the TCGCPUOps::guest_default_memory_order field and have
each target initialize it.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Anton Johansson <anjo@rev.ng>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>