docs: Add @defbuiltin family of helpers

The @defbuiltin{,x} macros are convenience macros for the often-repeated
task of defining a built-in function in extend.texi.  Usage of this
macro should lead to a higher degree of consistency across pieces of
text written by different people, and provide a better reading
experience, as they prevent easy-to-make errors, like forgetting index
entries for these functions.

gcc/ChangeLog:

	* doc/include/gcc-common.texi: Add @defbuiltin{,x} and
	@enddefbuiltin for defining built-in functions.
	* doc/extend.texi: Apply @defbuiltin{,x} to many, but not all,
	places where it should be used.