[NDS32] Consider varargs situation when creating stack operation assembly code.

[NDS32] Consider varargs situation when creating stack operation assembly code.

  -- In fact, we only need to take care of 'push.s' situation,
     because we have to push all the nameless arguments into stack.
  -- We do not have to use 'pop.s' to restore them back to registers,
     because we can just adjust stack pointer to set frame location.
     This operation should be done in epilogue expanding.
  -- Because 'pop25' instruction also performs return operation, we will
     not use v3push/v3pop on isr and vararg functions.
     Therefore, their instruction length should be 4 bytes.

	* config/nds32/nds32-md-auxiliary.c (nds32_output_stack_push):
	Check rtx for varargs implementation.
	(nds32_output_stack_pop): Likewise.
	* config/nds32/nds32-protos.h: Have a rtx argument for
	nds32_output_stack_push and nds32_output_stack_pop.
	* config/nds32/nds32.md: Likewise.

From-SVN: r214856