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/cmath

@@ -37,6 +37,8 @@
 #include <cstddef>
 #include <math.h>
 
+extern "C++" {
+
 namespace std
 {
 // Functions.
@@ -68,4 +70,6 @@ using ::floor;
 using ::fmod;
 }  // namespace std
 
+}  // extern C++
+
 #endif  // BIONIC_LIBSTDCPP_INCLUDE_CMATH__