Fix hash field access for 64-bit big-endian architectures.

Need to use the kHashFieldSlot rather than kHashFieldOffset for
pointer-sized memory accesses.

(Fix for "[builtins] Migrate String.prototype.charCodeAt and String.prototype.charAt to TurboFan.")

R=bmeurer@chromium.org, epertoso@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1907393002

Cr-Commit-Position: refs/heads/master@{#35741}

src/code-stub-assembler.cc

@@ -609,7 +609,7 @@ Node* CodeStubAssembler::AllocateSeqOneByteString(int length) {
   StoreMapNoWriteBarrier(result, LoadRoot(Heap::kOneByteStringMapRootIndex));
   StoreObjectFieldNoWriteBarrier(result, SeqOneByteString::kLengthOffset,
                                  SmiConstant(Smi::FromInt(length)));
-  StoreObjectFieldNoWriteBarrier(result, SeqOneByteString::kHashFieldOffset,
+  StoreObjectFieldNoWriteBarrier(result, SeqOneByteString::kHashFieldSlot,
                                  IntPtrConstant(String::kEmptyHashField));
   return result;
 }
@@ -619,7 +619,7 @@ Node* CodeStubAssembler::AllocateSeqTwoByteString(int length) {
   StoreMapNoWriteBarrier(result, LoadRoot(Heap::kStringMapRootIndex));
   StoreObjectFieldNoWriteBarrier(result, SeqTwoByteString::kLengthOffset,
                                  SmiConstant(Smi::FromInt(length)));
-  StoreObjectFieldNoWriteBarrier(result, SeqTwoByteString::kHashFieldOffset,
+  StoreObjectFieldNoWriteBarrier(result, SeqTwoByteString::kHashFieldSlot,
                                  IntPtrConstant(String::kEmptyHashField));
   return result;
 }