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_arm.cc b/src/context_arm.cc
index 3c2af94..387b71d 100644
--- a/src/context_arm.cc
+++ b/src/context_arm.cc
@@ -32,6 +32,7 @@
 }
 
 void ArmContext::DoLongJump() {
+#if defined(__arm__)
   // TODO: Load all GPRs and FPRs, currently the code restores registers R4 to PC
   asm volatile ( "mov %%r0, %0\n"
                  "mov %%r1, %1\n"
@@ -50,6 +51,9 @@
         "w28"(fprs_[28]), "w29"(fprs_[29]), "w30"(fprs_[30]), "w31"(fprs_[31])
 #endif
       :);  // clobber
+#else
+  UNIMPLEMENTED(FATAL);
+#endif
 }
 
 }  // namespace arm