libstdc++: Install <coroutine> header for freestanding [PR103726]

The standard says that <coroutine> should be present for freestanding.
That was intentionally left out of the initial implementation, but can
be done without much trouble. The header should be moved to libsupc++ at
some point in stage 1.

The standard also says that <coroutine> defines a std::hash
specialization, which was missing from our implementation. That's a
problem for freestanding (see LWG 3653) so only do that for hosted.

We can use concepts to constrain the __coroutine_traits_impl base class
when compiled with concepts enabled. In a pure C++20 implementation we
would not need that base class at all and could just use a constrained
partial specialization of coroutine_traits. But the absence of the
__coroutine_traits_impl<R, void> base would create an ABI difference
between the non-standard C++14/C++17 support for coroutines and the same
code compiled as C++20. If we drop support for <coroutine> pre-C++20 we
should revisit this.

libstdc++-v3/ChangeLog:

	PR libstdc++/103726
	* include/Makefile.am: Install <coroutine> for freestanding.
	* include/Makefile.in: Regenerate.
	* include/std/coroutine: Adjust headers and preprocessor
	conditions.
	(__coroutine_traits_impl): Use concepts when available.
	[_GLIBCXX_HOSTED] (hash<coroutine_handle>): Define.

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