libstdc++: Move C++14 <chrono> components to new <bits/chrono.h> header

This moves the "classic" contents of <chrono> to a new header, so that
<future>, <thread> etc. can get use durations and clocks without
calendar types, time zones, and chrono I/O.

libstdc++-v3/ChangeLog:

	* include/Makefile.am: Add new header.
	* include/Makefile.in: Regenerate.
	* include/std/chrono (duration, time_point, system_clock)
	(steady_clock, high_resolution_clock, chrono_literals, sys_time)
	(file_clock, file_time): Move to ...
	* include/bits/chrono.h: New file.
	* include/bits/atomic_futex.h: Include new header instead of
	<chrono>.
	* include/bits/atomic_timed_wait.h: Likewise.
	* include/bits/fs_fwd.h: Likewise.
	* include/bits/semaphore_base.h: Likewise.
	* include/bits/this_thread_sleep.h: Likewise.
	* include/bits/unique_lock.h: Likewise.
	* include/experimental/bits/fs_fwd.h: Likewise.
	* include/experimental/chrono: Likewise.
	* include/experimental/io_context: Likewise.
	* include/experimental/netfwd: Likewise.
	* include/experimental/timer: Likewise.
	* include/std/condition_variable: Likewise.
	* include/std/mutex: Likewise.
	* include/std/shared_mutex: Likewise.