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/context_x86.cc b/src/context_x86.cc
index 2f328e1..43de4ba 100644
--- a/src/context_x86.cc
+++ b/src/context_x86.cc
@@ -2,16 +2,22 @@
 
 #include "context_x86.h"
 
+#include "logging.h"
+
 namespace art {
 namespace x86 {
 
 void X86Context::DoLongJump() {
+#if defined(__i386__)
   // Load ESP and EIP
   asm volatile ( "movl %%esp, %0\n"
                  "jmp *%1"
       : // output
       : "m"(esp_), "r"(&eip_)  // input
       :);  // clobber
+#else
+  UNIMPLEMENTED(FATAL);
+#endif
 }
 
 }  // namespace x86