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/mpz/export.c b/third_party/gmp/mpz/export.c
new file mode 100644
index 0000000..2dacd69
--- /dev/null
+++ b/third_party/gmp/mpz/export.c
@@ -0,0 +1,189 @@
+/* mpz_export -- create word data from mpz.
+
+Copyright 2002, 2003, 2012 Free Software Foundation, Inc.
+
+This file is part of the GNU MP Library.
+
+The GNU MP Library is free software; you can redistribute it and/or modify
+it under the terms of either:
+
+  * the GNU Lesser General Public License as published by the Free
+    Software Foundation; either version 3 of the License, or (at your
+    option) any later version.
+
+or
+
+  * the GNU General Public License as published by the Free Software
+    Foundation; either version 2 of the License, or (at your option) any
+    later version.
+
+or both in parallel, as here.
+
+The GNU MP Library 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 copies of the GNU General Public License and the
+GNU Lesser General Public License along with the GNU MP Library.  If not,
+see https://www.gnu.org/licenses/.  */
+
+#include <stdio.h>  /* for NULL */
+#include "gmp-impl.h"
+#include "longlong.h"
+
+
+#if HAVE_LIMB_BIG_ENDIAN
+#define HOST_ENDIAN     1
+#endif
+#if HAVE_LIMB_LITTLE_ENDIAN
+#define HOST_ENDIAN     (-1)
+#endif
+#ifndef HOST_ENDIAN
+static const mp_limb_t  endian_test = (CNST_LIMB(1) << (GMP_LIMB_BITS-7)) - 1;
+#define HOST_ENDIAN     (* (signed char *) &endian_test)
+#endif
+
+void *
+mpz_export (void *data, size_t *countp, int order,
+	    size_t size, int endian, size_t nail, mpz_srcptr z)
+{
+  mp_size_t      zsize;
+  mp_srcptr      zp;
+  size_t         count, dummy;
+  unsigned long  numb;
+  unsigned       align;
+
+  ASSERT (order == 1 || order == -1);
+  ASSERT (endian == 1 || endian == 0 || endian == -1);
+  ASSERT (nail <= 8*size);
+  ASSERT (nail <  8*size || SIZ(z) == 0); /* nail < 8*size+(SIZ(z)==0) */
+
+  if (countp == NULL)
+    countp = &dummy;
+
+  zsize = SIZ(z);
+  if (zsize == 0)
+    {
+      *countp = 0;
+      return data;
+    }
+
+  zsize = ABS (zsize);
+  zp = PTR(z);
+  numb = 8*size - nail;
+  MPN_SIZEINBASE_2EXP (count, zp, zsize, numb);
+  *countp = count;
+
+  if (data == NULL)
+    data = (*__gmp_allocate_func) (count*size);
+
+  if (endian == 0)
+    endian = HOST_ENDIAN;
+
+  align = ((char *) data - (char *) NULL) % sizeof (mp_limb_t);
+
+  if (nail == GMP_NAIL_BITS)
+    {
+      if (size == sizeof (mp_limb_t) && align == 0)
+	{
+	  if (order == -1 && endian == HOST_ENDIAN)
+	    {
+	      MPN_COPY ((mp_ptr) data, zp, (mp_size_t) count);
+	      return data;
+	    }
+	  if (order == 1 && endian == HOST_ENDIAN)
+	    {
+	      MPN_REVERSE ((mp_ptr) data, zp, (mp_size_t) count);
+	      return data;
+	    }
+
+	  if (order == -1 && endian == -HOST_ENDIAN)
+	    {
+	      MPN_BSWAP ((mp_ptr) data, zp, (mp_size_t) count);
+	      return data;
+	    }
+	  if (order == 1 && endian == -HOST_ENDIAN)
+	    {
+	      MPN_BSWAP_REVERSE ((mp_ptr) data, zp, (mp_size_t) count);
+	      return data;
+	    }
+	}
+    }
+
+  {
+    mp_limb_t      limb, wbitsmask;
+    size_t         i, numb;
+    mp_size_t      j, wbytes, woffset;
+    unsigned char  *dp;
+    int            lbits, wbits;
+    mp_srcptr      zend;
+
+    numb = size * 8 - nail;
+
+    /* whole bytes per word */
+    wbytes = numb / 8;
+
+    /* possible partial byte */
+    wbits = numb % 8;
+    wbitsmask = (CNST_LIMB(1) << wbits) - 1;
+
+    /* offset to get to the next word */
+    woffset = (endian >= 0 ? size : - (mp_size_t) size)
+      + (order < 0 ? size : - (mp_size_t) size);
+
+    /* least significant byte */
+    dp = (unsigned char *) data
+      + (order >= 0 ? (count-1)*size : 0) + (endian >= 0 ? size-1 : 0);
+
+#define EXTRACT(N, MASK)                                \
+    do {                                                \
+      if (lbits >= (N))                                 \
+        {                                               \
+          *dp = limb MASK;                              \
+          limb >>= (N);                                 \
+          lbits -= (N);                                 \
+        }                                               \
+      else                                              \
+        {                                               \
+          mp_limb_t  newlimb;                           \
+          newlimb = (zp == zend ? 0 : *zp++);           \
+          *dp = (limb | (newlimb << lbits)) MASK;       \
+          limb = newlimb >> ((N)-lbits);                \
+          lbits += GMP_NUMB_BITS - (N);                 \
+        }                                               \
+    } while (0)
+
+    zend = zp + zsize;
+    lbits = 0;
+    limb = 0;
+    for (i = 0; i < count; i++)
+      {
+	for (j = 0; j < wbytes; j++)
+	  {
+	    EXTRACT (8, + 0);
+	    dp -= endian;
+	  }
+	if (wbits != 0)
+	  {
+	    EXTRACT (wbits, & wbitsmask);
+	    dp -= endian;
+	    j++;
+	  }
+	for ( ; j < size; j++)
+	  {
+	    *dp = '\0';
+	    dp -= endian;
+	  }
+	dp += woffset;
+      }
+
+    ASSERT (zp == PTR(z) + ABSIZ(z));
+
+    /* low byte of word after most significant */
+    ASSERT (dp == (unsigned char *) data
+	    + (order < 0 ? count*size : - (mp_size_t) size)
+	    + (endian >= 0 ? (mp_size_t) size - 1 : 0));
+  }
+  return data;
+}