MDEV-6473 - main.statistics fails on PPC64

mysql.column_stats wasn't stored/restored properly on big-endian
with histogram_type=DOUBLE_PREC_HB.

Store histogram values using int2store()/uint2korr().

Note that this patch invalidates previously calculated histogram
values on big-endian.

sql/item_strfunc.cc

@@ -602,7 +602,7 @@ String *Item_func_decode_histogram::val_str(String *str)
       val= p[i] / ((double)((1 << 8) - 1));
       break;
     case DOUBLE_PREC_HB:
-      val= ((uint16 *)(p + i))[0] / ((double)((1 << 16) - 1));
+      val= uint2korr(p + i) / ((double)((1 << 16) - 1));
       i++;
       break;
     default:

sql/sql_statistics.h

@@ -147,7 +147,7 @@ class Histogram
     case SINGLE_PREC_HB:
       return (uint) (((uint8 *) values)[i]);
     case DOUBLE_PREC_HB:
-      return (uint) (((uint16 *) values)[i]);
+      return (uint) uint2korr(values + i * 2);
     }
     return 0;
   }
@@ -214,7 +214,7 @@ class Histogram
       ((uint8 *) values)[i]= (uint8) (val * prec_factor());
       return;
     case DOUBLE_PREC_HB:
-      ((uint16 *) values)[i]= (uint16) (val * prec_factor());
+      int2store(values + i * 2, val * prec_factor());
       return;
     }
   }
@@ -226,7 +226,7 @@ class Histogram
       ((uint8 *) values)[i]= ((uint8 *) values)[i-1];
       return;
     case DOUBLE_PREC_HB:
-      ((uint16 *) values)[i]= ((uint16 *) values)[i-1];
+      int2store(values + i * 2, uint2korr(values + i * 2 - 2));
       return;
     }
   }