use crypto/sha256 only for FIPS 140-2 compliance (#14983)

It would seem like the PR #11623 had chewed more
than it wanted to, non-fips build shouldn't really
be forced to use slower crypto/sha256 even for
presumed "non-performance" codepaths. In MinIO
there are really no "non-performance" codepaths.
This assumption seems to have had an adverse
effect in certain areas of CPU usage.

This PR ensures that we stick to sha256-simd
on all non-FIPS builds, our most common build
to ensure we get the best out of the CPU at
any given point in time.

internal/hash/reader.go

@@ -26,6 +26,7 @@ import (
 	"io"
 
 	"github.com/minio/minio/internal/etag"
+	"github.com/minio/minio/internal/hash/sha256"
 )
 
 // A Reader wraps an io.Reader and computes the MD5 checksum
@@ -122,7 +123,7 @@ func NewReader(src io.Reader, size int64, md5Hex, sha256Hex string, actualSize i
 	}
 	var hash hash.Hash
 	if len(SHA256) != 0 {
-		hash = newSHA256()
+		hash = sha256.New()
 	}
 	return &Reader{
 		src:           src,