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 riscv#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)