lib/hash: Use CRC32 for hashing.

Use CRC32 intrinsics for hash computations when building for
X86_64 with SSE4_2.

Add a new hash_words64() and change hash_words() to be inlined when
'n_words' is a compile-time constant.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>