sys: tell compiler about impossible aliasing in dlist APIs

The compiler was inserting additional redundant loads in many
`sys_dlist_*` APIs, in case writes aliased with previous reads.

However, these additional reads are unnecessary, as the only cases
where the aliasing would matter would be a violation of the `dlist`
API contract (e.g. if node->next == node but node->prev != node).

This is decidedly a micro-optimization.

Signed-off-by: James Harris <james.harris@intel.com>