crc32c: choose function in static initialization

Before: 4.430 micros/op 225732 ops/sec; 881.8 MB/s (4K per op)
After: 4.125 micros/op 242425 ops/sec; 947.0 MB/s (4K per op)
Merge pull request #71 from alberts/crc32

crc32c: choose function in static initialization