LE_LOAD64 is only used when _mm_crc32_u64 is available, on 64-bit x86 processors.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=148906169

This change authored by vadimskipin and submitted via:

    https://github.com/google/leveldb/pull/309

Changes made to support iOS builds and other architectures
without support for SSE 4.2.

db_bench reports original crc32 speed at:

    crc32c : 3.610 micros/op; 1082.0 MB/s (4K per op)

with this change performance has increased to:

    crc32c : 0.843 micros/op; 4633.6 MB/s (4K per op)

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=148694935