libstdc++: Encapsulate __gthread_cond_t as std::__condvar

This introduces a new internal utility, std::__condvar, which is a
simplified form of std::condition_variable. It has no dependency on
<chrono> or std::unique_lock, which allows it to be used in
<bits/atomic_wait.h>.

This avoids repeating the #ifdef __GTHREAD_COND_INIT preprocessor
conditions and associated logic for initializing a __gthread_cond_t
correctly. It also encapsulates most of the __gthread_cond_xxx functions
as member functions of __condvar.

libstdc++-v3/ChangeLog:

	* include/bits/atomic_timed_wait.h (__cond_wait_until_impl):
	Do not define when _GLIBCXX_HAVE_LINUX_FUTEX is defined. Use
	__condvar and mutex instead of __gthread_cond_t and
	unique_lock<mutex>.
	(__cond_wait_until): Likewise. Fix test for return value of
	__cond_wait_until_impl.
	(__timed_waiters::_M_do_wait_until): Use __condvar instead
	of __gthread_cond_t.
	* include/bits/atomic_wait.h: Remove <bits/unique_lock.h>
	include. Only include <bits/std_mutex.h> if not using futexes.
	(__platform_wait_max_value): Remove unused variable.
	(__waiters::lock_t): Use lock_guard instead of unique_lock.
	(__waiters::_M_cv): Use __condvar instead of __gthread_cond_t.
	(__waiters::_M_do_wait(__platform_wait_t)): Likewise.
	(__waiters::_M_notify()): Likewise. Use notify_one() if not
	asked to notify all.
	* include/bits/std_mutex.h (__condvar): New type.
	* include/std/condition_variable (condition_variable::_M_cond)
	(condition_variable::wait_until): Use __condvar instead of
	__gthread_cond_t.
	* src/c++11/condition_variable.cc (condition_variable): Define
	default constructor and destructor as defaulted.
	(condition_variable::wait, condition_variable::notify_one)
	(condition_variable::notify_all): Forward to corresponding
	member function of __condvar.

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