Add ldn/stn single support, fix ldnr support.

Add ldn/stn single support, fix ldnr support.

	sim/aarch64/
	* simulator.c: (LDn_STn_SINGLE_LANE_AND_SIZE): New.
	(do_vec_LDn_single, do_vec_STn_single): New.
	(do_vec_LDnR): Add and set new nregs var.  Replace switch on nregs with
	loop over nregs using new var n.  Add n times size to address in loop.
	Add n to vd in loop.
	(do_vec_load_store): Add comment for instruction bit 24.  New var
	single to hold instruction bit 24.  Add new code to use single.  Move
	ldnr support inside single if statements.  Fix ldnr register counts
	inside post if statement.  Change HALT_NYI calls to HALT_UNALLOC.

	sim/testsuite/sim/aarch64/
	* ldn_single.s: New.
	* ldnr.s: New.
	* stn_single.s: New.