Move ACCESS_ONCE() to <linux/compiler.h>

It actually makes much more sense there, and we do tend to need it for
non-RCU usage too.  Moving it to <linux/compiler.h> will allow some
other cases that have open-coded the same logic to use the same helper
function that RCU has used.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>