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/cstdlib b/libstdc++/include/cstdlib
index 5e6a0b3..e520543 100644
--- a/libstdc++/include/cstdlib
+++ b/libstdc++/include/cstdlib
@@ -35,6 +35,8 @@
  */
 #include <stdlib.h>
 
+extern "C++" {
+
 namespace std {
 
 using ::exit;
@@ -110,4 +112,6 @@
 using ::wcstombs;
 }  // namespace std
 
+}  // extern C++
+
 #endif  // BIONIC_LIBSTDCPP_INCLUDE_CSTDLIB__