Shuffle fields to make simarm and arm agree on the offset of Heap::new_space_.to_.

runtime/vm/atomic_linux.h

Index: runtime/vm/atomic_linux.h
diff --git a/runtime/vm/atomic_linux.h b/runtime/vm/atomic_linux.h
index 615004a5647aabfa41f0b6a55d54afa71016a108..3e6f4adde07ce2cc00ab70090d58f2d8594d3728 100644
--- a/runtime/vm/atomic_linux.h
+++ b/runtime/vm/atomic_linux.h
@@ -27,7 +27,12 @@ inline void AtomicOperations::IncrementBy(intptr_t* p, intptr_t value) {
 
 
 inline void AtomicOperations::IncrementInt64By(int64_t* p, int64_t value) {
+#if defined(TARGET_ARCH_MIPS)
+  // No double-word atomics on MIPS32.
+  *p += value;
+#else
   __sync_fetch_and_add(p, value);
+#endif
 }