[13/77] Make floatn_mode return an opt_scalar_float_mode

[13/77] Make floatn_mode return an opt_scalar_float_mode

As per subject.

2017-08-30  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* target.def (default_floatn_mode): Return an opt_scalar_float_mode.
	* doc/tm.texi: Regenerate.
	* config/arm/arm.c (arm_floatn_mode): Return an opt_scalar_float_mode.
	* config/powerpcspe/powerpcspe.c (rs6000_floatn_mode): Likewise.
	* config/rs6000/rs6000.c (rs6000_floatn_mode): Likewise.
	* targhooks.h (default_floatn_mode): Likewise.
	* targhooks.c (default_floatn_mode): Likewise.
	* tree.c (build_common_tree_nodes): Update accordingly.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r251465