[committed] libstdc++: Ensure C++20 std::stringstream definitions use correct ABI

[Jonathan Wakely <jwakely@redhat.com>]

Tested powerpc64le-linux (old ABI) and x86_64-linux (new ABI), pushed to
trunk.


The definitions of the new C++20 members of std::stringstream etc are
missing when --with-default-libstdcxx-abi=gcc4-compatible is used,
because all the explicit instantiations in src/c++20/sstream-inst.cc are
skipped.

This ensures the contents of that file are compiled with the new ABI, so
the same set of symbols are exported regardless of which ABI is active
by default.

libstdc++-v3/ChangeLog:

	* src/c++20/sstream-inst.cc (_GLIBCXX_USE_CXX11_ABI): Define to
	select new ABI.
---
 libstdc++-v3/src/c++20/sstream-inst.cc | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/src/c++20/sstream-inst.cc b/libstdc++-v3/src/c++20/sstream-inst.cc
index b3fbd6ead44..55d1fe5234d 100644
--- a/libstdc++-v3/src/c++20/sstream-inst.cc
+++ b/libstdc++-v3/src/c++20/sstream-inst.cc
@@ -26,7 +26,9 @@
 // ISO C++ 14882:
 //
 
-// Instantiations in this file are only for the new SSO std::string ABI
+// Instantiations in this file are only for the new SSO std::string ABI.
+#define _GLIBCXX_USE_CXX11_ABI 1
+
 #include <sstream>
 
 #if _GLIBCXX_USE_CXX11_ABI
-- 
2.31.1