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.

libstdc++-v3/include/std/iostream

@@ -73,7 +73,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   ///@}
 
   // For construction of filebuffers for cout, cin, cerr, clog et. al.
+  // When the init_priority attribute is usable, we do this initialization
+  // in the compiled library instead (src/c++98/ios_init.cc).
+#if !__has_attribute(__init_priority__)
   static ios_base::Init __ioinit;
+#endif
 
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace

libstdc++-v3/src/c++98/ios_base_init.h

@@ -0,0 +1,12 @@
+// This is only in a header so we can use the system_header pragma,
+// to suppress the warning caused by using a reserved init_priority.
+#pragma GCC system_header
+
+// If the target supports init priorities, set up a static object in the
+// compiled library to perform the <iostream> initialization once and
+// sufficiently early (so that it happens before any other global
+// constructor when statically linking with libstdc++.a), instead of
+// doing so in (each TU that includes) <iostream>.
+#if __has_attribute(init_priority)
+static ios_base::Init __ioinit __attribute__((init_priority(90)));
+#endif

libstdc++-v3/src/c++98/ios_init.cc

@@ -75,6 +75,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   extern wostream wclog;
 #endif
 
+#include "ios_base_init.h"
+
   ios_base::Init::Init()
   {
     if (__gnu_cxx::__exchange_and_add_dispatch(&_S_refcount, 1) == 0)