[NDS32] new attribute no_prologue and new option -mret-in-naked-func.

[NDS32] new attribute no_prologue and new option -mret-in-naked-func.

gcc/
	* config/nds32/nds32.c (nds32_attribute_table): Add "no_prologue".
	(nds32_init_machine_status): Initialize machine->attr_naked_p and
	machine->attr_no_prologue_p.
	(nds32_compute_stack_frame): Check "naked" and "no_prologue" attributes.
	(nds32_naked_function_p): Handle "naked" and "no_prologue" attributes.
	(nds32_expand_epilogue): Consider attr_naked_p.
	(nds32_expand_epilogue_v3pop): Likewise.
	(nds32_can_use_return_insn): Likewise.
	* config/nds32/nds32.h (machine_function): Add attr_naked_p and
	attr_no_prologue_p fields.
	* config/nds32/nds32.opt (mret-in-naked-func): New option.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@260803 138bc75d-0d04-0410-961f-82ee72b054a4