Add libgmp 6.2.0 to third_party

Don't build it yet.  That will come in the next review.

Change-Id: Idf3266558165e5ab45f4a41c98cc8c838c8244d5
diff --git a/third_party/gmp/tests/devel/tst-addsub.c b/third_party/gmp/tests/devel/tst-addsub.c
new file mode 100644
index 0000000..4aa827a
--- /dev/null
+++ b/third_party/gmp/tests/devel/tst-addsub.c
@@ -0,0 +1,97 @@
+/* Copyright 1996, 2001 Free Software Foundation, Inc.
+
+This file is part of the GNU MP Library test suite.
+
+The GNU MP Library test suite is free software; you can redistribute it
+and/or modify it under the terms of the GNU General Public License as
+published by the Free Software Foundation; either version 3 of the License,
+or (at your option) any later version.
+
+The GNU MP Library test suite is distributed in the hope that it will be
+useful, but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General
+Public License for more details.
+
+You should have received a copy of the GNU General Public License along with
+the GNU MP Library test suite.  If not, see https://www.gnu.org/licenses/.  */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include "gmp-impl.h"
+#include "tests.h"
+
+#define ADD 1
+#define SUB 2
+
+#ifndef METHOD
+#define METHOD ADD
+#endif
+
+#if METHOD == ADD
+#define REFCALL refmpn_add_n
+#define TESTCALL mpn_add_n
+#endif
+
+#if METHOD == SUB
+#define REFCALL refmpn_sub_n
+#define TESTCALL mpn_sub_n
+#endif
+
+#define SIZE 100
+
+int
+main (int argc, char **argv)
+{
+  mp_size_t alloc_size, max_size, size, i, cumul_size;
+  mp_ptr s1, s2, dx, dy;
+  int s1_align, s2_align, d_align;
+  long pass, n_passes;
+  mp_limb_t cx, cy;
+
+  max_size = SIZE;
+  n_passes = 1000000;
+
+  argc--; argv++;
+  if (argc)
+    {
+      max_size = atol (*argv);
+      argc--; argv++;
+    }
+
+  alloc_size = max_size + 32;
+  s1 = malloc (alloc_size * GMP_LIMB_BYTES);
+  s2 = malloc (alloc_size * GMP_LIMB_BYTES);
+  dx = malloc (alloc_size * GMP_LIMB_BYTES);
+  dy = malloc (alloc_size * GMP_LIMB_BYTES);
+
+  cumul_size = 0;
+  for (pass = 0; pass < n_passes; pass++)
+    {
+      size = random () % max_size + 1;
+
+      cumul_size += size;
+      if (cumul_size >= 1000000)
+	{
+	  cumul_size -= 1000000;
+	  printf ("\r%ld", pass); fflush (stdout);
+	}
+      s1_align = random () % 32;
+      s2_align = random () % 32;
+      d_align = random () % 32;
+
+      mpn_random2 (s1 + s1_align, size);
+      mpn_random2 (s2 + s2_align, size);
+
+      for (i = 0; i < alloc_size; i++)
+	dx[i] = dy[i] = i + 0x9876500;
+
+      cx = TESTCALL (dx + d_align, s1 + s1_align, s2 + s2_align, size);
+      cy = REFCALL (dy + d_align, s1 + s1_align, s2 + s2_align, size);
+
+      if (cx != cy || mpn_cmp (dx, dy, alloc_size) != 0)
+	abort ();
+    }
+
+  printf ("%ld passes OK\n", n_passes);
+  exit (0);
+}