[NDS32] Implement n8 pipeline.

[NDS32] Implement n8 pipeline.

gcc/
	* config.gcc (nds32*-*-*): Check that n6/n8/s8 are valid to --with-cpu.
	* config/nds32/nds32-n8.md: New file.
	* config/nds32/nds32-opts.h (nds32_cpu_type): Add CPU_N6 and CPU_N8.
	* config/nds32/nds32-pipelines-auxiliary.c: Implementation for n8
	pipeline.
	* config/nds32/nds32-protos.h: More declarations for n8 pipeline.
	* config/nds32/nds32-utils.c: More implementations for n8 pipeline.
	* config/nds32/nds32.md (pipeline_model): Add n8.
	* config/nds32/nds32.opt (mcpu): Support n8 pipeline cpus.
	* config/nds32/pipelines.md: Include n8 settings.

Co-Authored-By: Chung-Ju Wu <jasonwucj@gmail.com>
From-SVN: r259219