ARC: Increase readability of entry handlers

* use artificial PUSH/POP contructs for CORE Reg save/restore to stack
* use artificial PUSHAX/POPAX contructs for Auxiliary Space regs
* macro'ize multiple copies of callee-reg-save/restore (SAVE_R13_TO_R24)
* use BIC insn for inverse-and operation

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/kernel/process.c

@@ -73,6 +73,10 @@ asmlinkage void ret_from_fork(void);
  * ~                ~
  * |    --to--      |   (scratch Regs of user mode)
  * |     r0         |
+ * ------------------
+ * |      SP        |
+ * |    orig_r0     |
+ * |    orig_r8     |
  * ------------------  <===== END of PAGE
  */
 int copy_thread(unsigned long clone_flags,