[SDL] Atomic int/ptr operations?

[Ryan C. Gordon]

> Mac OS X: 
> http://developer.apple.com/documentation/Cocoa/Conceptual/Multithreading/ThreadSafety/ThreadSafety.html#//apple_ref/doc/uid/10000057i-CH8-SW14

You should use those on PowerPC builds, because it's hard to get it 
right in assembly (and possibly impossible across all PowerPC chips).

But on Intel chips, you might as well just use the same "lock ; xchg" 
opcodes everyone else does, instead of relying on the OS.

--ryan.