[utils] Lower kMinComplexMemCopy on 64-bit archs to 8

Down from 16 * kPointerSize.
Modern compilers have good inlining support for memcpy(), so our
custom C++ loop is only beneficial for very short loops (if at all).

BUG=v8:5395

Review-Url: https://codereview.chromium.org/2438583002
Cr-Commit-Position: refs/heads/master@{#43212}

src/utils.h

@@ -508,7 +508,7 @@ V8_EXPORT_PRIVATE V8_INLINE void MemMove(void* dest, const void* src,
                                          size_t size) {
   memmove(dest, src, size);
 }
-const int kMinComplexMemCopy = 16 * kPointerSize;
+const int kMinComplexMemCopy = 8;
 #endif  // V8_TARGET_ARCH_IA32