Define std::gcd and std::lcm for C++17

Define std::gcd and std::lcm for C++17

	* doc/xml/manual/status_cxx2017.xml: Update gcd/lcm status.
	* doc/html/*: Regenerate.
	* include/experimental/numeric (__abs): Move to <numeric>.
	(gcd, lcm): Use __detail::gcd and __detail::lcm.
	* include/std/numeric (__detail::__abs_integral)
	(__detail::__gcd, __detail::__lcm): Define.
	(gcd, lcm): Define for C++17.
	* testsuite/26_numerics/gcd/1.cc: New test.
	* testsuite/26_numerics/lcm/1.cc: New test.
	* testsuite/experimental/numeric/gcd.cc: Swap contents with ...
	* testsuite/experimental/numeric/lcd.cc: ... this.

From-SVN: r240723