libstdc++: Enable most of <chrono> for freestanding

This makes durations, time points and calendrical types available for
freestanding. The clocks and time zone utilities are disabled for
freestanding, as they require functions in the hosted lib.

Add support for a new macro _GLIBCXX_NO_FREESTANDING_CHRONO which can be
used to explicitly disable <chrono> for freestanding.

libstdc++-v3/ChangeLog:

	* doc/xml/manual/using.xml (_GLIBCXX_NO_FREESTANDING_CHRONO):
	Document macro.
	* doc/html/*: Regenerate.
	* include/bits/chrono.h [_GLIBCXX_NO_FREESTANDING_CHRONO]:
	Only include <bits/require_hosted.h> when this macro is defined.
	[_GLIBCXX_HOSTED]: Only define clocks for hosted.
	* include/bits/version.def (chrono_udls): Remove hosted=yes.
	* include/bits/version.h: Regenerate.
	* include/std/chrono [_GLIBCXX_HOSTED]: Only define clocks and
	time zone utilities for hosted.
	* testsuite/std/time/freestanding.cc: New test.