RISC-V: Allow nested implications for extensions

Certain extensions require two levels of implications.  For example,
zvkng implies zvkn and zvkn implies zvkned.  Enabling zvkng should also
enable zvkned.

This patch fixes this behavior.

bfd/ChangeLog:

	* elfxx-riscv.c (riscv_parse_add_implicit_subsets): Allow nested
	implications for extensions.

Signed-off-by: Nathan Huckleberry <nhuck@google.com>
Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>

Author: nhukc

bfd/elfxx-riscv.c

@@ -1873,14 +1873,29 @@ static void
 riscv_parse_add_implicit_subsets (riscv_parse_subset_t *rps)
 {
   struct riscv_implicit_subset *t = riscv_implicit_subsets;
-  for (; t->subset_name; t++)
+  bool finished = false;
+  while (!finished)
     {
-      riscv_subset_t *subset = NULL;
-      if (riscv_lookup_subset (rps->subset_list, t->subset_name, &subset)
-	  && t->check_func (t->implicit_name, subset))
-	riscv_parse_add_subset (rps, t->implicit_name,
-				RISCV_UNKNOWN_VERSION,
-				RISCV_UNKNOWN_VERSION, true);
+      finished = true;
+      for (; t->subset_name; t++)
+	{
+	  riscv_subset_t *subset = NULL;
+	  riscv_subset_t *implicit_subset = NULL;
+	  if (riscv_lookup_subset (rps->subset_list, t->subset_name, &subset)
+	      && !riscv_lookup_subset (rps->subset_list, t->implicit_name,
+				       &implicit_subset)
+	      && t->check_func (t->implicit_name, subset))
+	    {
+	      riscv_parse_add_subset (rps, t->implicit_name,
+				      RISCV_UNKNOWN_VERSION,
+				      RISCV_UNKNOWN_VERSION, true);
+
+	      /* Restart the loop and pick up any new implications.  */
+	      finished = false;
+	      t = riscv_implicit_subsets;
+	      break;
+	    }
+	}
     }
 }