Issue #13156: revert changeset f6feed6ec3f9, which was only relevant for native TLS implementations, and fails with the ad-hoc TLS implementation when a thread doesn't have an auto thread state (e.g. a thread created outside of Python calling into a subinterpreter).

#include "Python.h"

#ifndef PLATFORM
#define PLATFORM "unknown"
#endif

const char *
Py_GetPlatform(void)
{
	return PLATFORM;
}