Add gc build tag for the asm code.

Fixes #27.

decode_amd64.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build gc
+
 package snappy
 
 // decode has the same semantics as in decode_other.go.

decode_amd64.s

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build gc
+
 #include "textflag.h"
 
 // func decode(dst, src []byte) int

decode_other.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !amd64
+// +build !amd64 !gc
 
 package snappy