libstdc++: Use <cstdint> instead of <stdint.h> in <bits/atomic_wait.h>

In r14-5922-g6c8f2d3a08bc01 I added <stdint.h> to <bits/atomic_wait.h>,
so that uintptr_t is declared if that header is compiled as a header
unit. I used <stdint.h> because that's what <atomic> already includes,
so it seemed simpler to be consistent. However, this means that name
lookup for uintptr_t in <bits/atomic_wait.h> depends on whether
<cstdint> has been included by another header first. Whether name lookup
finds std::uintptr_t or ::uintptr_t will depend on include order. This
causes problems when compiling modules with Clang:

bits/atomic_wait.h:251:7: error: 'std::__detail::__waiter_pool_base' has different definitions in different modules; first difference is defined here found method '_S_for' with body
      _S_for(const void* __addr) noexcept
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
bits/atomic_wait.h:251:7: note: but in 'tm.<global>' found method '_S_for' with different body
      _S_for(const void* __addr) noexcept
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

By including <cstdint> we would ensure that name lookup always finds the
name in namespace std. Alternatively, we can stop including <stdint.h>
for those types, so that we don't declare the entire contents of
<stdint.h> when we only need a couple of types from it. This patch does
the former, which is appropriate for backporting.

libstdc++-v3/ChangeLog:

	* include/bits/atomic_wait.h: Include <cstdint> instead of
	<stdint.h>.

(cherry picked from commit 2f512f6f)