Bluetooth: Controller: Duplicate and add aux offset to chain PDU in ULL

Refactor PDU chaining implementation to add aux offset in
the aux pointer field in the parent PDU when duplicating
chain PDUs.

By adding the aux offsets in the ULL execution context the
LLL prepare execution in Extended and Periodic Advertising
does not need to have a O(n) while loop in LLL execution
context.

This will reduce CPU use in the highest priority ISR
compared to current implementation where chain PDUs where
duplicated in ULL execution context and aux offset populated
in LLL execution context.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>