Harshavardhana 5878fcc086 bit-rot: Default to sha256 on ARM64. (#3488)
This is to utilize an optimized version of
sha256 checksum which @fwessels implemented.

blake2b lacks such optimizations on ARM platform,
this can provide us significant boost in performance.

blake2b on ARM64 as expected would be slower.
```
BenchmarkSize1K-4           	   30000	     44015 ns/op	  23.26 MB/s
BenchmarkSize8K-4           	    5000	    335448 ns/op	  24.42 MB/s
BenchmarkSize32K-4          	    1000	   1333960 ns/op	  24.56 MB/s
BenchmarkSize128K-4         	     300	   5328286 ns/op	  24.60 MB/s
```

sha256 on ARM64 is faster by orders of magnitude giving close to
AVX performance of blake2b.
```
BenchmarkHash8Bytes-4	 1000000	      1446 ns/op	   5.53 MB/s
BenchmarkHash1K-4    	  500000	      3229 ns/op	 317.12 MB/s
BenchmarkHash8K-4    	  100000	     14430 ns/op	 567.69 MB/s
BenchmarkHash1M-4    	    1000	   1640126 ns/op	 639.33 MB/s
```
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