libstdc++: Move stream initialization into compiled library [PR44952]

libstdc++: Move stream initialization into compiled library [PR44952]

This patch moves the static object for constructing the standard streams
out from <iostream> and into the compiled library on systems that support
init priorities.  This'll mean <iostream> no longer introduces a separate
global constructor in each TU that includes it.

We can do this only if the init_priority attribute is supported because
we need a way to ensure the stream initialization runs first before any
user global initializer, particularly when linking with a static
libstdc++.a.

	PR libstdc++/44952
	PR libstdc++/39796
	PR libstdc++/98108

libstdc++-v3/ChangeLog:

	* include/std/iostream (__ioinit): No longer define here if
	the init_priority attribute is usable.
	* src/c++98/ios_init.cc (__ioinit): Define here instead if
	init_priority is usable, via ...
	* src/c++98/ios_base_init.h: ... this new file.