libstdc++: Use std::chrono::steady_clock as atomic_futex reference clock

The user-visible effect of this change is that std::future::wait_for now
uses std::chrono::steady_clock to determine the timeout.  This makes it
immune to changes made to the system clock.  It also means that anyone
using their own clock types with std::future::wait_until will have the
timeout converted to std::chrono::steady_clock rather than
std::chrono::system_clock.

Now that use of both std::chrono::steady_clock and
std::chrono::system_clock are correctly supported for the wait timeout, I
believe that std::chrono::steady_clock is a better choice for the reference
clock that all other clocks are converted to since it is guaranteed to
advance steadily.  The previous behaviour of converting to
std::chrono::system_clock risks timeouts changing dramatically when the
system clock is changed.

libstdc++-v3/ChangeLog:

	* include/bits/atomic_futex.h (__atomic_futex_unsigned): Change
	__clock_t typedef to use steady_clock so that unknown clocks are
	synced to it rather than system_clock. Change existing __clock_t
	overloads of _M_load_and_text_until_impl and
	_M_load_when_equal_until to use system_clock explicitly. Remove
	comment about DR 887 since these changes address that problem as
	best as we currently able.

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