GCM: use counter mode code for speed-up
GCM: Tweak Intel PCLMUL ghash loop for small speed-up

#ifdef __x86_64__
      if (nblocks >= 4)
        {
          do
            {
              buf += 4 * blocksize;
              nblocks -= 4;
            }
          while (nblocks >= 4);
        }
#endif
Add aggregated bulk processing for GCM on x86-64

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Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>
GCM: Add stack burning
GCM: Add clearing and checking of marks.tag