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.

cmd/signature-v4.go

@@ -27,7 +27,6 @@ package cmd
 
 import (
 	"bytes"
-	"crypto/sha256"
 	"crypto/subtle"
 	"encoding/hex"
 	"net/http"
@@ -40,6 +39,7 @@ import (
 	"github.com/minio/minio-go/v7/pkg/s3utils"
 	"github.com/minio/minio-go/v7/pkg/set"
 	"github.com/minio/minio/internal/auth"
+	"github.com/minio/minio/internal/hash/sha256"
 	xhttp "github.com/minio/minio/internal/http"
 )