libstdc++: stdc++.h and <coroutine>

libstdc++: stdc++.h and <coroutine>

r13-3036 moved #include <coroutine> into the new freestanding section, but
also moved it from a C++20 section to a C++23 section.  This patch moves it
back.

Incidentally, I'm curious why a few headers were removed from the hosted
section (including <coroutine>), but most were left in place, so we have
redundant includes of most hosted headers.

libstdc++-v3/ChangeLog:

	* include/precompiled/stdc++.h: <coroutine> is C++20.