Add new options: -mext-perf, -mext-perf2, -mext-string.

Add new options: -mext-perf, -mext-perf2, -mext-string.

gcc/
	* config/nds32/nds32.opt: Add mext-perf, mext-perf2, mext-string.
	* config/nds32/nds32.opt: Refine the layout.
	* config/nds32/nds32.c (TARGET_EXT_PERF, TARGET_EXT_PERF2,
	TARGET_EXT_STRING): Support new options.
	* config/nds32/nds32.h: Likewise.
	* config/nds32/nds32.md: Likewise.
	* config/nds32/nds32-predicates.c: Likewise.
	* config/nds32/constraints.md: Likewise.
	* common/config/nds32/nds32-common.c: Likewise.

Co-Authored-By: Kito Cheng <kito.cheng@gmail.com>
From-SVN: r254798