libstdc++ Fix compilation of <stop_token> with Clang

Clang 9 supports C++20 via -std=c++2a but doesn't support three-way
comparisons, so <stop_token> fails to compile. When the compiler doesn't
support default comparisons, this patch defines operator== and
operator!= for the _Stop_state_ref class. That is enough for the header
to be compiled with Clang. It allows operator== for stop_token and
stop_source to work, but not operator!= because that isn't explicitly
defined.

	* include/std/stop_token (stop_token::_Stop_state_ref): Define
	comparison operators explicitly if the compiler won't synthesize them.

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