extend.texi (Declaring Attributes of Functions): Split into subsections by target.

extend.texi (Declaring Attributes of Functions): Split into subsections by target.

2015-04-28  Sandra Loosemore  <sandra@codesourcery.com>

	gcc/
	* doc/extend.texi (Declaring Attributes of Functions): Split into
	subsections by target.  Alphabetize the table of common attributes.
	Rewrite some of the introductory text to reflect the new structure.
	Update some cross-references to point to the new subsections.
	(Attribute Syntax): Put paragraph about "__" naming here.  Remove
	duplicate copies in the discussion of function, label, and type
	attributes.

From-SVN: r222543