Doc: Rearrange remaining top-level sections in extend.texi [PR42270]

Doc: Rearrange remaining top-level sections in extend.texi [PR42270]

This is part of an incremental effort to make the chapter on GCC
extensions better organized by grouping/rearranging sections by topic.

gcc/ChangeLog
	PR other/42270
	* doc/extend.texi (Nonlocal Gotos): Group with other built-ins
	sections.
	(Constructing Calls): Likewise.
	(Pragmas): Move earlier in the section, before the built-ins docs.
	(Thread-Local): Likewise.
	(OpenMP): Likewise.
	(OpenACC): Likewise.