Add WT_STATIC_ASSERT calls to the atomic operation macros so we don't choose the wrong macro.

@markbenvenuto, @michaelcahill: I have exactly no confidence we'll not change the size of some structure field and get the choice of macro wrong, so I've added `WT_STATIC_ASSERT` calls to the gcc versions of these macros. We might want to do the same thing for Windows (although, obviously, if gcc and MSVC use different sizes for the fields, we will need different macro names, but at least we'll catch the failure).

I stared at this for awhile, and couldn't think of anything else that might work that wasn't hugely ugly and/or invasive, I'd love to hear a better idea.