Fix wrong endianness of wasm header in WasmModuleWriter on big-endian platforms

BUG=mjsunit/wasm/asm-wasm-f*,mjsunit/wasm/asm-wasm-i*

Review-Url: https://codereview.chromium.org/2013393002
Cr-Commit-Position: refs/heads/master@{#36548}

src/wasm/encoder.h

@@ -36,13 +36,25 @@ class ZoneBuffer : public ZoneObject {
 
   void write_u16(uint16_t x) {
     EnsureSpace(2);
+#if V8_TARGET_LITTLE_ENDIAN
     WriteUnalignedUInt16(pos_, x);
+#else
+    pos_[0] = x & 0xff;
+    pos_[1] = (x >> 8) & 0xff;
+#endif
     pos_ += 2;
   }
 
   void write_u32(uint32_t x) {
     EnsureSpace(4);
+#if V8_TARGET_LITTLE_ENDIAN
     WriteUnalignedUInt32(pos_, x);
+#else
+    pos_[0] = x & 0xff;
+    pos_[1] = (x >> 8) & 0xff;
+    pos_[2] = (x >> 16) & 0xff;
+    pos_[3] = (x >> 24) & 0xff;
+#endif
     pos_ += 4;
   }