Fix C file generation in jit.bcsave.

Thanks to codicodi.

src/jit/bcsave.lua

@@ -119,7 +119,7 @@ local function bcsave_c(ctx, output, s)
   local fp = savefile(output, "w")
   if ctx.type == "c" then
     fp:write(string.format([[
-#ifdef _cplusplus
+#ifdef __cplusplus
 extern "C"
 #endif
 #ifdef _WIN32