libstdc++: Move std::thread to a new header

This makes it possible to use std::thread without including the whole of
<thread>. It also makes this_thread::get_id() and this_thread::yield()
available even when there is no gthreads support (e.g. when GCC is built
with --disable-threads or --enable-threads=single).

In order for the std::thread::id return type of this_thread::get_id() to
be defined, std:thread itself is defined unconditionally. However the
constructor that creates new threads is not defined for single-threaded
builds. The thread::join() and thread::detach() member functions are
defined inline for single-threaded builds and just throw an exception
(because we know the thread cannot be joinable if the constructor that
creates joinable threads doesn't exit).

The thread::hardware_concurrency() member function is also defined
inline and returns 0 (as suggested by the standard when the value "is
not computable or well-defined").

The main benefit for most targets is that other headers such as <future>
do not need to include the whole of <thread> just to be able to create a
std::thread. That avoids including <stop_token> and std::jthread where
not required. This is another partial fix for PR 92546.

This also means we can use this_thread::get_id() and this_thread::yield()
in <stop_token> instead of using the gthread functions directly. This
removes some preprocessor conditionals, simplifying the code.

libstdc++-v3/ChangeLog:

	PR libstdc++/92546
	* include/Makefile.am: Add new <bits/std_thread.h> header.
	* include/Makefile.in: Regenerate.
	* include/std/future: Include new header instead of <thread>.
	* include/std/stop_token: Include new header instead of
	<bits/gthr.h>.
	(stop_token::_S_yield()): Use this_thread::yield().
	(_Stop_state_t::_M_requester): Change type to std::thread::id.
	(_Stop_state_t::_M_request_stop()): Use this_thread::get_id().
	(_Stop_state_t::_M_remove_callback(_Stop_cb*)): Likewise.
	Use __is_single_threaded() to decide whether to synchronize.
	* include/std/thread (thread, operator==, this_thread::get_id)
	(this_thread::yield): Move to new header.
	(operator<=>, operator!=, operator<, operator<=, operator>)
	(operator>=, hash<thread::id>, operator<<): Define even when
	gthreads not available.
	* src/c++11/thread.cc: Include <memory>.
	* include/bits/std_thread.h: New file.
	(thread, operator==, this_thread::get_id, this_thread::yield):
	Define even when gthreads not available.
	[!_GLIBCXX_HAS_GTHREADS] (thread::join, thread::detach)
	(thread::hardware_concurrency): Define inline.