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

libstdc++/include/cstddef

@@ -38,9 +38,13 @@
  */
 #include <stddef.h>
 
+extern "C++" {
+
 namespace std {
 using ::ptrdiff_t;
 using ::size_t;
 }  // namespace std
 
+}  // extern C++
+
 #endif  // BIONIC_LIBSTDCPP_INCLUDE_CSTDDEF__