libstdc++: Pass __wait_args to internal API by const pointer

libstdc++: Pass __wait_args to internal API by const pointer

This change splits the __wait_args data members to a new struct
__wait_args_base and then passes that type by const pointer to the low
level implementation functions.

libstdc++-v3/ChangeLog:

	* include/bits/atomic_timed_wait.h (__spin_until_impl): Accept
	__wait_args as const __wait_args_base*.
	(__wait_until_impl): Likewise.
	(__wait_until): Likewise.
	(__wait_for): Likewise.
	(__atomic_wait_address_until): Pass __wait_args by address.
	(__atomic_wait_address_for): Likewise.
	* include/bits/atomic_wait.h (__wait_args_base): New struct.
	(__wait_args): Derive from __wait_args_base.
	(__wait_args::__wait_args()): Adjust ctors to call call base ctor.
	(__wait_args::__wait_args(const __wait_args_base&)): New ctor.
	(__wait_args::operator|=): New method.
	(__wait_args::_S_flags_for): Change return type to
	__wait_flags.
	(__spin_impl): Accept __wait_args as const __wait_args_base*.
	(__wait_impl): Likewise.
	(__notify_impl): Likewise.
	(__atomic_wait_address): Pass __wait_args by address.
	(__atomic_wait_address_v): Likewise.
	(__atomic_notify_address): Likewise.