Fix ldn/stn multiple instructions. Fix testcases with unaligned data.
sim/aarch64/
* simulator.c (vec_load): Add M argument. Rewrite to iterate over
registers based on structure size.
(LD4, LD3, LD2, LD1_2, LD1_3, LD1_4): Pass new arg to vec_load.
(LD1_1): Replace with call to vec_load.
(vec_store): Add new M argument. Rewrite to iterate over registers
based on structure size.
(ST4, ST3, ST2, ST1_2, ST1_3, ST1_4): Pass new arg to vec_store.
(ST1_1): Replace with call to vec_store.