Don't read 8 bytes for 4-byte PTEs

For Sv32 Page Table Entries are only 4 bytes, but the old code was unconditionally reading 8 bytes.

Fixes #459

model/riscv_vmem.sail

@@ -107,14 +107,17 @@ function pt_walk(sv_params,
  let mem_result = mem_read_priv(Read(Data), // AccessType
  Supervisor, // Privilege
  pte_phys_addr,
- 8, // atom (8)
+ 2 ^ sv_params.log_pte_size_bytes,
  false, // aq
  false, // rl
  false); // res
 
  match mem_result {
  MemException(_) => PTW_Failure(PTW_Access(), ext_ptw),
  MemValue(pte) => {
+ // Extend to 64 bits even on RV32 for simplicity.
+ let pte : bits(64) = zero_extend(pte);
+
  let pte_flags = Mk_PTE_Flags(pte[7 .. 0]);
  if pte_is_invalid(pte_flags) then
  PTW_Failure(PTW_Invalid_PTE(), ext_ptw)