More work on making port use newlib's reentry mechanisms

- archContextSwitch stores address of new thread's struct reent in
  ctx_switch_info
- pend_sv_handler uses address in  ctx_switch_info.reent instead of fixed
  offset into atom_tcb
- added header with defines for offsets into struct ctx_switch_info used
  in assembler code. Also added compile time verification for this offsets

kernel/atom.h

@@ -59,10 +59,7 @@ typedef struct atom_tcb
      */
     POINTER sp_save_ptr;
 
-    /* Thread's port specific private data. Do not move, some poorly written
-     * thread switching code (*cough* Cortex-M *cough*) might depend on a 
-     * known offset into the atom_tcb struct
-     */
+    /* Thread's port specific private data. */
     THREAD_PORT_PRIV;
 
     /* Thread priority (0-255) */