[wasm][fuzzer] Support functions returning i64

Those functions will return a BigInt, which we need to convert to int to
match the value returned by the interpreter.

R=ahaas@chromium.org

Bug: chromium:1127717
Change-Id: I328660ab73776de2bd90b19d18e46663efe6b6cb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2412177Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69922}

src/objects/bigint.h

@@ -210,7 +210,7 @@ class BigInt : public BigIntBase {
   static MaybeHandle<BigInt> FromWords64(Isolate* isolate, int sign_bit,
                                          int words64_count,
                                          const uint64_t* words);
-  int64_t AsInt64(bool* lossless = nullptr);
+  V8_EXPORT_PRIVATE int64_t AsInt64(bool* lossless = nullptr);
   uint64_t AsUint64(bool* lossless = nullptr);
   int Words64Count();
   void ToWordsArray64(int* sign_bit, int* words64_count, uint64_t* words);

test/common/wasm/wasm-module-runner.cc

@@ -231,6 +231,9 @@ int32_t CallWasmFunctionForTesting(Isolate* isolate,
   if (result->IsHeapNumber()) {
     return static_cast<int32_t>(HeapNumber::cast(*result).value());
   }
+  if (result->IsBigInt()) {
+    return static_cast<int32_t>(BigInt::cast(*result).AsInt64());
+  }
   return -1;
 }