threads: Move stack_size to end of struct thread (followup to f19ffe2)

Makes the structure more compact.

Fixes the build failure on the m68k M5 target

Change-Id: If17b872f4923d3a52b344e3c59ed6f870d5a7725

firmware/kernel/thread-internal.h

@@ -126,14 +126,14 @@ struct thread_entry
                                   creation or thread_set_priority) */
     unsigned char priority;      /* Scheduled priority (higher of base or
                                     all threads blocked by this one) */
-#endif
-#ifndef HAVE_SDL_THREADS
-    size_t stack_size;           /* Size of stack in bytes */
 #endif
     unsigned char state;         /* Thread slot state (STATE_*) */
 #ifdef HAVE_SCHEDULER_BOOSTCTRL
     unsigned char cpu_boost;     /* CPU frequency boost flag */
 #endif
+#ifndef HAVE_SDL_THREADS
+    size_t stack_size;           /* Size of stack in bytes */
+#endif
 };
 
 /* Thread ID, 32 bits = |VVVVVVVV|VVVVVVVV|VVVVVVVV|SSSSSSSS| */