extend.texi (Variable Attributes): Add menu and proper @nodes to subsections.

2015-05-01  Sandra Loosemore  <sandra@codesourcery.com>

	gcc/
	* doc/extend.texi (Variable Attributes): Add menu and proper
	@nodes to subsections.  Move Microsoft Windows attributes to
	their own subsection.
	(Type Attributes): Reorganize introduction to remove duplicate
	list of attributes.  Add menu and proper @nodes to subsections.
	Alphabetize the main table of common attributes.

From-SVN: r222714