goto-gcc: run original compiler even when output is /dev/null

regression/goto-gcc/dev_null1/main.c

@@ -0,0 +1,4 @@
+void fails(void)
+{
+  asm volatile ( ".if (0 == 0); .error \"asm error\";.endif" );
+}

regression/goto-gcc/dev_null1/test.desc

@@ -0,0 +1,8 @@
+CORE
+main.c
+-o /dev/null
+^EXIT=1$
+^SIGNAL=0$
+--
+^warning: ignoring
+^CONVERSION ERROR$

src/goto-cc/gcc_mode.cpp

@@ -914,7 +914,7 @@ int gcc_modet::gcc_hybrid_binary(compilet &compiler)
   if(output_files.empty() ||
      (output_files.size()==1 &&
       output_files.front()=="/dev/null"))
-    return EX_OK;
+    return run_gcc(compiler);
 
   debug() << "Running " << native_tool_name
           << " to generate hybrid binary" << eom;