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/csetjmp b/libstdc++/include/csetjmp
index a68d127..ba82144 100644
--- a/libstdc++/include/csetjmp
+++ b/libstdc++/include/csetjmp
@@ -36,6 +36,8 @@
 
 #include <setjmp.h>
 
+extern "C++" {
+
 #ifndef setjmp
 #define setjmp(env) setjmp (env)
 #endif
@@ -46,4 +48,6 @@
 using ::longjmp;
 }  // namespace std
 
+}  // extern C++
+
 #endif  // BIONIC_LIBSTDCPP_INCLUDE_CSETJMP__