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
```

cmd/xl-v1-metadata.go

@@ -20,6 +20,7 @@ import (
 	"encoding/json"
 	"errors"
 	"path"
+	"runtime"
 	"sort"
 	"sync"
 	"time"
@@ -54,9 +55,28 @@ type checkSumInfo struct {
 }
 
 // Constant indicates current bit-rot algo used when creating objects.
-const (
-	bitRotAlgo = "blake2b"
-)
+// Depending on the architecture we are choosing a different checksum.
+var bitRotAlgo = getDefaultBitRotAlgo()
+
+// Get the default bit-rot algo depending on the architecture.
+// Currently this function defaults to "blake2b" as the preferred
+// checksum algorithm on all architectures except ARM64. On ARM64
+// we use sha256 (optimized using sha2 instructions of ARM NEON chip).
+func getDefaultBitRotAlgo() string {
+	switch runtime.GOARCH {
+	case "arm64":
+		// As a special case for ARM64 we use an optimized
+		// version of hash i.e sha256. This is done so that
+		// blake2b is sub-optimal and slower on ARM64.
+		// This would also allows erasure coded writes
+		// on ARM64 servers to be on-par with their
+		// counter-part X86_64 servers.
+		return "sha256"
+	default:
+		// Default for all other architectures we use blake2b.
+		return "blake2b"
+	}
+}
 
 // erasureInfo - carries erasure coding related information, block
 // distribution and checksums.