[NDS32] Implment n15 pipeline.

[NDS32] Implment n15 pipeline.

gcc/
	* config.gcc (nds32*-*-*): Check that n15 is valid to --with-cpu.
	* config/nds32/nds32-graywolf.md: New file.
	* config/nds32/nds32-opts.h (nds32_cpu_type): Add CPU_GRAYWOLF.
	* config/nds32/nds32-pipelines-auxiliary.c: Implementation for n15
	pipeline.
	* config/nds32/nds32-protos.h: More declarations for n15 pipeline.
	* config/nds32/nds32-utils.c: More implementations for n15 pipeline.
	* config/nds32/nds32.md (pipeline_model): Add graywolf.
	* config/nds32/nds32.opt (mcpu): Support n15 pipeline cpus.
	* config/nds32/pipelines.md: Include n15 settings.

Co-Authored-By: Chung-Ju Wu <jasonwucj@gmail.com>
From-SVN: r260214