libstdc++: Use lock-free type for __platform_wait_t

libstdc++: Use lock-free type for __platform_wait_t

For non-futex targets the __platform_wait_t type is currently uint64_t,
but that requires a lock in libatomic for some 32-bit targets. We don't
really need a 64-bit type, so use unsigned long if that is lock-free,
and int otherwise. This should mean it's lock-free on a wider set of
targets.

libstdc++-v3/ChangeLog:

	* include/bits/atomic_wait.h (__detail::__platform_wait_t):
	Define as unsigned long if always lock-free, and unsigned int
	otherwise.