libstdc++: Optimize std::counting_semaphore for futex path

libstdc++: Optimize std::counting_semaphore for futex path

Rename __semaphore_base to __semaphore_impl, because it's not used as a
base class. Replace the three identical lambda expressions with a named
class, __semaphore_impl::_Available, which stores the most recent
value of the counter as a data member, and provides call operators that
test whether the value is decrementable (i.e. whether the semaphore can
be acquired).

Add a new __platform_semaphore_impl class template to be used when
__platform_wait is available, which uses __platform_wait_t for the
counter and uses more efficient atomic waits for the acquire functions.
For a binary semaphore some members are further optimized because we
know the counter can only be zero or one.

Also add a bare wait flag to __atomic_wait_address_v, for consistency
with __atomic_wait_address_until_v and __atomic_wait_address_for_v and
to allow semaphores to use it without the redundant overhead of tracking
waiters.

libstdc++-v3/ChangeLog:

	* include/bits/atomic_wait.h (__atomic_wait_address_v): Add bare
	wait flag.
	* include/bits/semaphore_base.h (__semaphore_base): Rename to
	__semaphore_impl. Replace local variable and predicate lambdas
	with _Available struct.
	(__platform_semaphore_impl): New class template.
	(__semaphore_impl): Remove alias template.
	(_Select_semaphore_impl): New alias template.
	* include/std/semaphore (counting_semaphore): Use
	_Select_semaphore_impl.