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/csignal b/libstdc++/include/csignal
index 2600a75..84f0e1d 100644
--- a/libstdc++/include/csignal
+++ b/libstdc++/include/csignal
@@ -36,6 +36,8 @@
 
 #include <signal.h>
 
+extern "C++" {
+
 namespace std
 {
 using ::sig_atomic_t;
@@ -43,4 +45,6 @@
 using ::raise;
 }  // namespace std
 
+}  // extern C++
+
 #endif  // BIONIC_LIBSTDCPP_INCLUDE_CSIGNAL__