PR libstdc++/82644 document IS 29124 support

PR libstdc++/82644 document IS 29124 support

Also fix declarations of special functions in C++17, to import them into
the global namespace in <math.h>, and to prevent defining the
non-standard hypergeometric functions in strict mode.

	PR libstdc++/82644
	* doc/xml/manual/intro.xml: Include new section.
	* doc/xml/manual/status_cxxis29124.xml: New section on IS 29124
	status.
	* include/bits/specfun.h [__STRICT_ANSI__] (hyperg, hypergf, hypergl)
	(conf_hyperg, conf_hypergf, conf_hypergl): Don't declare.
	* include/c_compatibility/math.h: Import special functions into
	global namespace for C++17.
	* testsuite/26_numerics/headers/cmath/82644.cc: New test.
	* testsuite/26_numerics/headers/cmath/functions_global_c++17.cc: New
	test.

From-SVN: r254004