Doc: Organize atomic memory builtins documentation [PR42270]

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

This installment adds a container section to hold documentation for
both the _atomic and _sync builtins, reordering them so that the new
_atomic interface is presented before the legacy _sync one.  I also
incorporated material from the separate x86 transactional memory
section directly into the __atomic builtins documentation instead of
retaining that as a parallel section.

gcc/ChangeLog
	PR other/42270
	* doc/extend.texi (Atomic Memory Access): New section.
	(__sync Builtins): Make it a subsection of the above.
	(Atomic Memory Access): Likewise.
	(x86 specific memory model extensions for transactional memory):
	Delete this section, incorporating the text into the discussion
	of __atomic builtins.

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