atomic: fix win32 atomic load const warnings

[sreimers]

No description provided.

[Lastique]

The load location was not const-qualified because _InterlockedCompareExchange64 could issue a store at that location, even though the stored value is unchanged. That is, you cannot perform a load from a read-only memory.

[sreimers]

I see, but I think we should follow the C11 definition, which defines const:

https://en.cppreference.com/w/c/atomic/atomic_load

atomic_load( const volatile A* obj );

Maybe better to drop the windows x86 __int64 atomic load workaround (with a compiler warning or something similar).

[Lastique]

It's for you to decide, of course, but I would rather keep the support for 64-bit atomics. IMHO, having those consistently across targets is more useful than being able to do atomic loads from read-only memory.

You can keep the const, of course, but I would add a note somewhere that this const does not mean support for read-only memory. It only means the operation doesn't change the value.