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/sha256block_linux.go

@@ -0,0 +1,39 @@
+// +build amd64
+
+//
+// Mini Object Storage, (C) 2015 Minio, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+package sha256
+
+// #cgo CFLAGS: -DHAS_SSE41 -DHAS_AVX -DHAS_AVX2
+// #include <stdint.h>
+// void sha256_transform_ssse3 (const char *input_data, uint32_t *digest, unsigned long num_blks);
+// void sha256_transform_avx (const char *input_data, uint32_t *digest, unsigned long num_blks);
+// void sha256_transform_rorx (const char *input_data, uint32_t *digest, unsigned long num_blks);
+import "C"
+import "unsafe"
+
+func blockSSE(dig *digest, p []byte) {
+	C.sha256_transform_ssse3((*C.char)(unsafe.Pointer(&p[0])), (*C.uint32_t)(unsafe.Pointer(&dig.h[0])), (C.ulong)(len(p)/64))
+}
+
+func blockAVX(dig *digest, p []byte) {
+	C.sha256_transform_avx((*C.char)(unsafe.Pointer(&p[0])), (*C.uint32_t)(unsafe.Pointer(&dig.h[0])), (C.ulong)(len(p)/64))
+}
+
+func blockAVX2(dig *digest, p []byte) {
+	C.sha256_transform_rorx((*C.char)(unsafe.Pointer(&p[0])), (*C.uint32_t)(unsafe.Pointer(&dig.h[0])), (C.ulong)(len(p)/64))
+}