libstdc++: use extern "C++" in all our C++ system headers.

This is needed to build an independent toolchain with g++ that doesn't think
that all these headers are in C.

Change-Id: Ie9a8ccfcab7780d6a4e5722777d61c2b1b312001
diff --git a/libstdc++/include/new b/libstdc++/include/new
index 19d8185..0253e8b 100644
--- a/libstdc++/include/new
+++ b/libstdc++/include/new
@@ -4,6 +4,8 @@
 
 #include <cstddef>
 
+extern "C++" {
+
 namespace std {
     struct nothrow_t {};
     extern const nothrow_t nothrow;
@@ -25,4 +27,6 @@
 inline void  operator delete(void*, void*) { }
 inline void  operator delete[](void*, void*) { }
 
+}  // extern C++
+
 #endif // __NEW__