Make it possible to enable tlab with a build parameter.

For testing.

Change-Id: I4d06a4db733b7fa09105f068028483cf88db7925
diff --git a/runtime/globals.h b/runtime/globals.h
index 0845475..ac8751c 100644
--- a/runtime/globals.h
+++ b/runtime/globals.h
@@ -109,6 +109,13 @@
 static constexpr bool kPoisonHeapReferences = false;
 #endif
 
+// If true, enable the tlab allocator by default.
+#ifdef ART_USE_TLAB
+static constexpr bool kUseTlab = true;
+#else
+static constexpr bool kUseTlab = false;
+#endif
+
 // Kinds of tracing clocks.
 enum class TraceClockSource {
   kThreadCpu,