Uses PCLMULQDQ instrucion for CRC computation. For text input, this change
sees about 5% speedup to compression at level of 3, 5, 6. Level-1/9 sees
significantly higher/lower speedup. Decompression shows about 20% speedup.

The file "contrib/amd64/crc32-pclmul_asm.S" is stolen from linux kerel 3.14
with minor change. In the near future, this file will be replaced with C
implementation.