libstdc++: Avoid including <cstdint> for std::char_traits

libstdc++: Avoid including <cstdint> for std::char_traits

We should prefer the __UINT_LEAST16_TYPE__ and __UINT_LEAST32_TYPE__
macros, if available, so that we don't need all of <cstdint> in every
header that uses std::char_traits.

libstdc++-v3/ChangeLog:

	* include/bits/char_traits.h: Only include <cstdint> when
	necessary.
	* include/std/stacktrace: Use __UINTPTR_TYPE__ instead of
	uintptr_t.
	* src/c++11/cow-stdexcept.cc: Include <stdint.h>.
	* src/c++17/floating_to_chars.cc: Likewise.
	* testsuite/20_util/assume_aligned/1.cc: Include <cstdint>.
	* testsuite/20_util/assume_aligned/3.cc: Likewise.
	* testsuite/20_util/shared_ptr/creation/array.cc: Likewise.