[libj6] Make sure thread stacks are aligned

Stacks were not 16-byte aligned when coming into j6::thread::init_proc,
make sure it aligns stacks for SSE by adding the `force_align_arg_pointer`
attribute.

src/libraries/j6/include/j6/thread.hh

@@ -69,6 +69,7 @@ public:
     thread(const thread&) = delete;
 
 private:
+    __attribute__ ((force_align_arg_pointer))
     static void init_proc(thread *t)
     {
         t->m_proc();