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/sha512/sha512block_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 sha512
+
+// #cgo CFLAGS: -DHAS_SSE41 -DHAS_AVX -DHAS_AVX2
+// #include <stdint.h>
+// void sha512_transform_ssse3 (const void* M, void* D, uint64_t L);
+// void sha512_transform_avx (const void* M, void* D, uint64_t L);
+// void sha512_transform_rorx (const void* M, void* D, uint64_t L);
+import "C"
+import "unsafe"
+
+func blockSSE(dig *digest, p []byte) {
+	C.sha512_transform_ssse3(unsafe.Pointer(&p[0]), unsafe.Pointer(&dig.h[0]), (C.uint64_t)(len(p)/chunk))
+}
+
+func blockAVX(dig *digest, p []byte) {
+	C.sha512_transform_avx(unsafe.Pointer(&p[0]), unsafe.Pointer(&dig.h[0]), (C.uint64_t)(len(p)/chunk))
+}
+
+func blockAVX2(dig *digest, p []byte) {
+	C.sha512_transform_rorx(unsafe.Pointer(&p[0]), unsafe.Pointer(&dig.h[0]), (C.uint64_t)(len(p)/chunk))
+}