This is a missing piece of the C++20 <chrono> header.

It would be good to move the code into the compiled library, so that we
don't need <sstream> in <chrono>. It could also use spanstream in C++20,
to avoid memory allocations. That can be changed at a later date.

libstdc++-v3/ChangeLog:

	* include/std/chrono (__detail::__units_suffix_misc): New
	helper function.
	(__detail::__units_suffix): Likewise.
	(chrono::operator<<(basic_ostream&, const duration&)): Define.
	* testsuite/20_util/duration/io.cc: New test.

Tested powerpc64le-linux. Committed to trunk.