Atomic instruction alignment coverpoints missing

Similar to the missing align coverpoints for loads/stores, atomics only test accesses at alignments of the native processor wordsize. For the *.w instructions on RV64, we need to test accessing the upper half of a 64 bit word (bit 3 being 1). No reason to make this exclusive to RV64, but that's where it is actually interesting coverage. When we eventually add support for the Zabha extension this will also extend to byte and halfword atomic accesses.