[wasm] Add a magic word and a version number to the binary.

src/wasm/wasm-macro-gen.h

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_WASM_MACRO_GEN_H_
 #define V8_WASM_MACRO_GEN_H_
 
 #include "src/wasm/wasm-opcodes.h"
 
 // Convenience macros for building Wasm bytecode directly into a byte array.
@@ skipping 247 matching lines @@
 #define WASM_F32_REINTERPRET_I32(x) kExprF32ReinterpretI32, x
 #define WASM_F64_SCONVERT_I32(x) kExprF64SConvertI32, x
 #define WASM_F64_UCONVERT_I32(x) kExprF64UConvertI32, x
 #define WASM_F64_SCONVERT_I64(x) kExprF64SConvertI64, x
 #define WASM_F64_UCONVERT_I64(x) kExprF64UConvertI64, x
 #define WASM_F64_CONVERT_F32(x) kExprF64ConvertF32, x
 #define WASM_F64_REINTERPRET_I64(x) kExprF64ReinterpretI64, x
 #define WASM_I32_REINTERPRET_F32(x) kExprI32ReinterpretF32, x
 #define WASM_I64_REINTERPRET_F64(x) kExprI64ReinterpretF64, x
 
+#define U32_LE(v)                                    \
+  static_cast<byte>(v), static_cast<byte>((v) >> 8), \
+      static_cast<byte>((v) >> 16), static_cast<byte>((v) >> 24)
+
+#define U16_LE(v) static_cast<byte>(v), static_cast<byte>((v) >> 8)
+
+#define WASM_MODULE_HEADER U32_LE(kWasmMagic), U32_LE(kWasmVersion)
+
+#define SIG_INDEX(v) U16_LE(v)
+#define FUNC_INDEX(v) U16_LE(v)
+#define NAME_OFFSET(v) U32_LE(v)
+
 #endif  // V8_WASM_MACRO_GEN_H_