cranelift: Fix implicit pointer argument register use (#5301)

* Fix arg handling to write to VRegs instead of physical regs

* Make is_included_in_clobbers required, and handle Args on x64 and riscv64

cranelift/codegen/src/machinst/mod.rs

@@ -104,9 +104,7 @@ pub trait MachInst: Clone + Debug {
     fn is_args(&self) -> bool;
 
     /// Should this instruction be included in the clobber-set?
-    fn is_included_in_clobbers(&self) -> bool {
-        true
-    }
+    fn is_included_in_clobbers(&self) -> bool;
 
     /// Generate a move.
     fn gen_move(to_reg: Writable<Reg>, from_reg: Reg, ty: Type) -> Self;