rs6000.c (rs6000_option_override_internal): Add comments to explain why certain error messages make mention of undocumented...

gcc/ChangeLog:

2016-07-22  Kelvin Nilsen  <kelvin@gcc.gnu.org>

	* config/rs6000/rs6000.c (rs6000_option_override_internal): Add
	comments to explain why certain error messages make mention of
	undocumented options.
	(rs6000_invalid_builtin): Change error messages to replace mention
	of undocumented options with mention of the -mcpu=power9 option
	that enables those undocumented options.
	* config/rs6000/rs6000.h (MASK_FLOAT128): New macro.
	(RS6000_BTM_FLOAT128): Use the new MASK_FLOAT128 macro in the
	definition of this macro to correct an existing error.
	* config/rs6000/rs6000.opt: Add the Undocumented qualifier to the
	mpower9-fusion, mpower9-vector, mpower9-dform, and mmodulo entries.
	* doc/extend.texi (PowerPC AltiVec Built-in Functions): Modify
	descriptions of built-in functions so that they depend on
	-mcpu=power9 instead of on the corresponding undocumented flags.
	* doc/invoke.texi (Option Summary):  Remove all mention of newly
	undocumented flags.
	(IBM RS/6000 and PowerPC Options): Likewise.
	* doc/md.texi (Constraints for Particuliar Machines): Remove all
	mention of newly undocumented flags.

From-SVN: r238648