[NDS32] Add intrinsic function for ffb, ffmism and flmism.

[NDS32] Add intrinsic function for ffb, ffmism and flmism.

gcc/
	* config/nds32/constants.md (unspec_element): Add UNSPEC_FFB,
	UNSPEC_FFMISM and UNSPEC_FLMISM.
	* config/nds32/nds32-intrinsic.c (bdesc_2arg): Add builtin description
	for ffb, ffmism and flmism.
	* config/nds32/nds32-intrinsic.md (unspec_ffb): Define new pattern.
	(unspec_ffmism): Ditto.
	(unspec_flmism): Ditto.
	(nds32_expand_builtin_impl): Check if string extension is available.
	* config/nds32/nds32.h (nds32_builtins): Add NDS32_BUILTIN_FFB,
	NDS32_BUILTIN_FFMISM and NDS32_BUILTIN_FLMISM.

Co-Authored-By: Chung-Ju Wu <jasonwucj@gmail.com>
Co-Authored-By: Kito Cheng <kito.cheng@gmail.com>
From-SVN: r258424