libstdc++: Document P1642 and extensions

libstdc++-v3/ChangeLog:

	* doc/xml/manual/using.xml: Document newly-freestanding
	headers and the effect of the -ffreestanding flag.
	* doc/xml/manual/status_cxx2023.xml: Document P1642R11 as
	completed.
	* doc/xml/manual/configure.xml: Document that hosted installs
	respect __STDC_HOSTED__.
	* doc/xml/manual/test.xml: Document how to run tests in
	freestanding mode.
	* doc/html/*: Regenerate.