[ELF] Add Config &Ctx::arg. NFC

And migrate LinkerDriver member functions to use `ctx.arg.x` instead of
`config->x`.

Author: MaskRay

lld/ELF/Config.h

@@ -544,6 +544,7 @@ struct InStruct {
 };
 
 struct Ctx {
+  Config &arg;
   LinkerDriver driver;
   LinkerScript *script;
   TargetInfo *target;