The original purpose of C was to write operating systems; at some point that was basically discarded, and it is now incredibly difficult to write an operating system safely in C anymore. It's like juggling lightsabers.
In practice, the memory model of real hardware systems do a much more thorough job of screwing you up than compiler optimizations, so any practical operating system already needs to build a memory model that can cope with hardware reordering. The only interaction you need with the compiler is a few hooks to stop optimizations, which are quite easy to build in (an empty volatile asm that clobbers memory will prevent reordering of memory operations, for example), and newer language standards actually give you specific mechanisms that give you the building blocks without the need to resort to inline asm tricks.