libstdc++: Implement std::formatter<std::thread::id> without <sstream> [PR115099]

The std::thread::id formatter uses std::basic_ostringstream without
including <sstream>, which went unnoticed because the test for it uses
a stringstream to check the output is correct.

The fix implemented here is to stop using basic_ostringstream for
formatting thread::id and just use std::format instead.

As a drive-by fix, the formatter specialization is constrained to
require that the thread::id::native_handle_type can be formatted, to
avoid making the formatter ill-formed if the pthread_t type is not a
pointer or integer. Since non-void pointers can't be formatted, ensure
that we convert pointers to const void* for formatting. Make a similar
change to the existing operator<< overload so that in the unlikely case
that pthread_t is a typedef for char* we don't treat it as a
null-terminated string when inserting into a stream.

libstdc++-v3/ChangeLog:

	PR libstdc++/115099
	* include/bits/std_thread.h: Declare formatter as friend of
	thread::id.
	* include/std/thread (operator<<): Convert non-void pointers to
	void pointers for output.
	(formatter): Add constraint that thread::native_handle_type is a
	pointer or integer.
	(formatter::format): Reimplement without basic_ostringstream.
	* testsuite/30_threads/thread/id/output.cc: Check output
	compiles before <sstream> has been included.

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