Add packing to class Thread.

Otherwise, code compiled on the host can use different offsets than
we need for the same structure on the target.

Also add commented-out code to start up the various GC daemon threads.
More Class native methods need to be implemented before that will work.

Change-Id: I618b647b92378eec1b25cee469c8cfccf42f21fd
diff --git a/src/macros.h b/src/macros.h
index 36afd27..9c5124b 100644
--- a/src/macros.h
+++ b/src/macros.h
@@ -123,4 +123,6 @@
 #define OFFSETOF_VOLATILE_MEMBER(t, f) \
   (reinterpret_cast<volatile char*>(&reinterpret_cast<t*>(16)->f) - reinterpret_cast<volatile char*>(16))
 
+#define PACKED __attribute__ ((__packed__))
+
 #endif  // ART_SRC_MACROS_H_