libstdc++: Deliver names of C functions in <stacktrace>

__cxa_demangle is only to demangle C++ names, for all C functions,
extern "C" functions, and including main it returns -2, in that case
just adapt the given name. Otherwise it's kept empty, which doesn't look
nice in the stacktrace.

libstdc++-v3/ChangeLog:

	* include/std/stacktrace (stacktrace_entry::_S_demangle): Use
	raw __name if __cxa_demangle could not demangle it.

Signed-off-by: Björn Schäpers <bjoern@hazardy.de>

libstdc++-v3/include/std/stacktrace

@@ -219,6 +219,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 					       &__status);
       if (__status == 0)
 	__s = __str;
+      else
+	__s = __name;
       __builtin_free(__str);
       return __s;
     }