Check misalignment of AMOs before address translation (#471)

This is optional according to the spec - you can check afterwards. However

1. it seems extremely unlikely that any real designs will do that for atomics, which (ignoring Zam which the model doesn't support yet), always have to be aligned, and
2. the LR and SC instructions already check before address translation, so this wasn't even consistent.

Ideally in future this would be configurable.

model/riscv_insts_aext.sail

@@ -180,7 +180,9 @@ function clause execute (AMO(op, aq, rl, rs2, rs1, width, rd)) = {
  match ext_data_get_addr(rs1, zeros(), ReadWrite(Data, Data), width_bytes) {
  Ext_DataAddr_Error(e) => { ext_handle_data_check_error(e); RETIRE_FAIL },
  Ext_DataAddr_OK(vaddr) => {
- match translateAddr(vaddr, ReadWrite(Data, Data)) {
+ if not(is_aligned(vaddr, width))
+ then { handle_mem_exception(vaddr, E_SAMO_Addr_Align()); RETIRE_FAIL }
+ else match translateAddr(vaddr, ReadWrite(Data, Data)) {
  TR_Failure(e, _) => { handle_mem_exception(vaddr, e); RETIRE_FAIL },
  TR_Address(addr, _) => {
  let eares = mem_write_ea(addr, width_bytes, aq & rl, rl, true);