v8: Fix unaligned access when deserializing snapshots.

The code was already careful to use memcpy() here, but then it
added needless casts to wider types that made the compiler think
that it can do 4-byte-aligned accesses when it couldn't.

(It's also a bug that the snapshot got loaded at an unaligned
address, but we can fix both bugs.)

BUG=chromium:729059

Review-Url: https://codereview.chromium.org/2915323002
Cr-Commit-Position: refs/heads/master@{#45698}

src/snapshot/serializer-common.h

@@ -271,13 +271,12 @@ class SerializedData {
 
  protected:
   void SetHeaderValue(int offset, uint32_t value) {
-    uint32_t* address = reinterpret_cast<uint32_t*>(data_ + offset);
-    memcpy(reinterpret_cast<uint32_t*>(address), &value, sizeof(value));
+    memcpy(data_ + offset, &value, sizeof(value));
   }
 
   uint32_t GetHeaderValue(int offset) const {
     uint32_t value;
-    memcpy(&value, reinterpret_cast<int*>(data_ + offset), sizeof(value));
+    memcpy(&value, data_ + offset, sizeof(value));
     return value;
   }