Thread-local allocation stack.

With this change, Ritz MemAllocTest gets ~14% faster on N4.

Bug: 9986565
Change-Id: I2fb7d6f7c5daa63dd4fc73ba739e6ae4ed820617
diff --git a/runtime/thread.cc b/runtime/thread.cc
index 9797a48..3382811 100644
--- a/runtime/thread.cc
+++ b/runtime/thread.cc
@@ -963,7 +963,9 @@
       thread_local_start_(nullptr),
       thread_local_pos_(nullptr),
       thread_local_end_(nullptr),
-      thread_local_objects_(0) {
+      thread_local_objects_(0),
+      thread_local_alloc_stack_top_(nullptr),
+      thread_local_alloc_stack_end_(nullptr) {
   CHECK_EQ((sizeof(Thread) % 4), 0U) << sizeof(Thread);
   state_and_flags_.as_struct.flags = 0;
   state_and_flags_.as_struct.state = kNative;