clang lacks proper gas support, implement stubs for sha256,sha512 for darwin.

REF: http://llvm.org/bugs/show_bug.cgi?id=18918

pkg/utils/crypto/sha256/sha256_darwin.go

@@ -0,0 +1,31 @@
+package sha256
+
+import (
+	"io"
+
+	"crypto/sha256"
+)
+
+// Sum256 - single caller sha256 helper
+func Sum256(data []byte) []byte {
+	d := sha256.New()
+	d.Write(data)
+	return d.Sum(nil)
+}
+
+// Sum - io.Reader based streaming sha256 helper
+func Sum(reader io.Reader) ([]byte, error) {
+	d := sha256.New()
+	var err error
+	for err == nil {
+		length := 0
+		byteBuffer := make([]byte, 1024*1024)
+		length, err = reader.Read(byteBuffer)
+		byteBuffer = byteBuffer[0:length]
+		d.Write(byteBuffer)
+	}
+	if err != io.EOF {
+		return nil, err
+	}
+	return d.Sum(nil), nil
+}

pkg/utils/crypto/sha256/sha256_linux.go

@@ -152,11 +152,11 @@ func (d *digest) checkSum() [Size]byte {
 /// Convenience functions
 
 // Sum256 - single caller sha256 helper
-func Sum256(data []byte) [Size]byte {
+func Sum256(data []byte) []byte {
 	var d digest
 	d.Reset()
 	d.Write(data)
-	return d.checkSum()
+	return d.Sum(nil)
 }
 
 // Sum - io.Reader based streaming sha256 helper