Fix a few warnings/errors for compiling with -fno-exceptions.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@178267 91177308-0d34-0410-b5e6-96231b3b80d8

src/system_error.cpp

@@ -195,6 +195,9 @@ __throw_system_error(int ev, const char* what_arg)
 {
 #ifndef _LIBCPP_NO_EXCEPTIONS
     throw system_error(error_code(ev, system_category()), what_arg);
+#else
+    (void)ev;
+    (void)what_arg;
 #endif
 }