libstdc++: Remove unnecessary uses of <stdint.h>

We don't need to include all of <stdint.h> when we only need uintptr_t
from it. By using GCC's internal macro we avoid unnecessarily declaring
everything in <stdint.h>. This helps users to avoid accidentally relying
on those names being declared without explicitly including the header.

libstdc++-v3/ChangeLog:

	* include/bits/align.h (align, assume_aligned): Use
	__UINTPTR_TYPE__ instead of uintptr_t. Do not include
	<stdint.h>.
	* include/bits/atomic_base.h (__atomic_ref): Likewise.
	* include/bits/atomic_wait.h (__waiter_pool_base::_S_for):
	Likewise.
	* include/std/atomic: Include <cstdint>.