crc32: suppress -Wimplicit-fallthrough warnings

crc32: suppress -Wimplicit-fallthrough warnings

Summary:
Workaround a bunch of "implicit-fallthrough" compiler errors, like:

```
util/crc32c.cc:533:7: error: this statement may fall through [-Werror=implicit-fallthrough=]
   crc = _mm_crc32_u64(crc, *(uint64_t*)(buf + offset));
       ^
util/crc32c.cc:1016:9: note: in expansion of macro ‘CRCsinglet’
         CRCsinglet(crc0, next, -2 * 8);
         ^~~~~~~~~~
util/crc32c.cc:1017:7: note: here
       case 1:
```
Closes https://github.com/facebook/rocksdb/pull/3339

Reviewed By: sagar0

Differential Revision: D6874736

Pulled By: quark-zju

fbshipit-source-id: eec9f3bc135e12fca336928d01711006d5c3cb16