libstdc++: Add preprocessor conditions for freestanding [PR106953]

libstdc++: Add preprocessor conditions for freestanding [PR106953]

This adds checks for _GLIBCXX_HOSTED to a number of headers which are
not currently installed for freestanding, but need to be for P1642R11
support. For example, <iterator> needs to be installed for C++23
freestanding mode, but without stream iterators and streambuf iterators.
Similarly, <memory> needs to be installed, but without std::allocator
and std::shared_ptr. This change disables the non-freestanding parts of
those headers.

libstdc++-v3/ChangeLog:

	PR libstdc++/106953
	* include/backward/auto_ptr.h [!_GLIBCXX_HOSTED]: Do not define
	shared_ptr members.
	* include/bits/alloc_traits.h [!_GLIBCXX_HOSTED]: Do not declare
	std::allocator_traits<std::allocator<T>> specializations for
	freestanding.
	* include/bits/memoryfwd.h [!_GLIBCXX_HOSTED] (allocator): Do
	not declare for freestanding.
	* include/bits/stl_algo.h [!_GLIBCXX_HOSTED] (stable_partition):
	Do not define for freestanding.
	[!_GLIBCXX_HOSTED] (merge, stable_sort): Do not use temporary
	buffers for freestanding.
	* include/bits/stl_algobase.h [!_GLIBCXX_HOSTED]: Do not declare
	streambuf iterators and overloaded algorithms using them.
	* include/bits/stl_uninitialized.h [!_GLIBCXX_HOSTED]: Do not
	define specialized overloads for std::allocator.
	* include/bits/unique_ptr.h [!_GLIBCXX_HOSTED] (make_unique)
	(make_unique_for_overwrite, operator<<): Do not define for
	freestanding.
	* include/c_global/cstdlib [!_GLIBCXX_HOSTED] (_Exit): Declare.
	Use _GLIBCXX_NOTHROW instead of throw().
	* include/debug/assertions.h [!_GLIBCXX_HOSTED]: Ignore
	_GLIBCXX_DEBUG for freestanding.
	* include/debug/debug.h [!_GLIBCXX_DEBUG]: Likewise.
	* include/std/bit [!_GLIBCXX_HOSTED]: Do not use the custom
	__int_traits if <ext/numeric_traits.h> is available.
	* include/std/functional [!_GLIBCXX_HOSTED]: Do not include
	headers that aren't valid for freestanding.
	(boyer_moore_searcher, boyer_moore_horspool_searcher): Do not
	define for freestanding.
	* include/std/iterator [!_GLIBCXX_HOSTED]: Do not include
	headers that aren't valid for freestanding.
	* include/std/memory [!_GLIBCXX_HOSTED]: Likewise.
	* include/std/ranges [!_GLIBCXX_HOSTED] (istream_view): Do not
	define for freestanding.
	(views::__detail::__is_basic_string_view) [!_GLIBCXX_HOSTED]:
	Do not define partial specialization for freestanding.