cache hash: Use sse4.2 CRC32c on x86-64 where available

This is more than twice as fast as adler32, but could be made another
50% faster by calculating crcs for 8 byte blocks in "parallel" (without
data dependency) and then combining them. But that's complicated code.

Reference measurements for hashing the cache 100 times:
 adler32=2.46s     xxhash64=0.64      xxhash32=1.12
 crc32c(this)=1.10 crc32c(opt)=0.44s

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