mttcg: warning to user about memory ordering

[Pierrick Bouvier]

Hello,

When using multi thread tcg, a warning is emitted to user if models for 
guest and host are not compatible (like qemu-system-x86_64 running on 
aarch64 host)

accel/tcg/tcg-all.c:
161             if (!check_tcg_memory_orders_compatible()) {
162                 warn_report("Guest expects a stronger memory ordering "
163                             "than the host provides");
164                 error_printf("This may cause strange/hard to debug 
errors\n");
165             }

However, in tcg/tcg-op.c, all load/stores are emitting memory barriers 
using tcg_gen_req_mo, which lets think it's already implemented for any 
host/guest combination (was added in b32dc3).

In your opinion, are there still corner cases that are not covered 
correctly? If no, should we drop this scary warning?

Regards,
Pierrick