RISC-V: Reorder the prefixed extensions which are out of order.

This patch has been pending for almost a year...  However, I noticed that
llvm can already re-order the extensions, even if they are out of orders.
Not really sure if they can also re-order the single letter extensions,
but at least we can do this for the multi-letter extensions in binutils.

bfd/
    * elfxx-riscv.c (riscv_parse_prefixed_ext): Removed the code which are
    used to check the prefixed extension orders.
gas/
    * testsuite/gas/riscv/march-fail-order-x-z.d: Removed since we will help
    tp reorder the prefixed extensions for now.
    * testsuite/gas/riscv/march-fail-order-x-z.l: Likewise.
    * testsuite/gas/riscv/march-fail-order-x.d: Likewise.
    * testsuite/gas/riscv/march-fail-order-x.l: Likewise.
    * testsuite/gas/riscv/march-fail-order-z.d: Likewise.
    * testsuite/gas/riscv/march-fail-order-z.l: Likewise.

bfd/elfxx-riscv.c

@@ -1729,7 +1729,6 @@ riscv_parse_prefixed_ext (riscv_parse_subset_t *rps,
 {
   int major_version;
   int minor_version;
-  const char *last_name;
   enum riscv_prefix_ext_class class;
 
   while (*p)
@@ -1806,28 +1805,6 @@ riscv_parse_prefixed_ext (riscv_parse_subset_t *rps,
 	  return NULL;
 	}
 
-      /* Check that the extension isn't duplicate.  */
-      last_name = rps->subset_list->tail->name;
-      if (!strcasecmp (last_name, subset))
-	{
-	  rps->error_handler
-	    (_("%s: duplicate prefixed ISA extension `%s'"),
-	     arch, subset);
-	  free (subset);
-	  return NULL;
-	}
-
-      /* Check that the extension is in expected order.  */
-      if (riscv_compare_subsets (last_name, subset) > 0)
-	{
-	  rps->error_handler
-	    (_("%s: prefixed ISA extension `%s' is not in expected "
-	       "order.  It must come before `%s'"),
-	     arch, subset, last_name);
-	  free (subset);
-	  return NULL;
-	}
-
       riscv_parse_add_subset (rps, subset,
 			      major_version,
 			      minor_version, false);