Aarch64: fix narrow integer-register extension with Baldrdash ABI.

In the Baldrdash (SpiderMonkey) embedding, we must take care to
zero-extend all function arguments to callees in integer registers when
the types are narrower than 64 bits. This is because, unlike the native
SysV ABI, the Baldrdash ABI expects high bits to be cleared. Not doing
so leads to difficult-to-trace errors where high bits falsely tag an
int32 as e.g. an object pointer, leading to potential security issues.

cranelift/codegen/src/machinst/abi.rs

@@ -1,7 +1,7 @@
 //! ABI definitions.
 
 use crate::binemit::Stackmap;
-use crate::ir::{ArgumentExtension, StackSlot};
+use crate::ir::StackSlot;
 use crate::machinst::*;
 use crate::settings;
 
@@ -52,12 +52,7 @@ pub trait ABIBody {
     fn gen_retval_area_setup(&self) -> Option<Self::I>;
 
     /// Generate an instruction which copies a source register to a return value slot.
-    fn gen_copy_reg_to_retval(
-        &self,
-        idx: usize,
-        from_reg: Writable<Reg>,
-        ext: ArgumentExtension,
-    ) -> Vec<Self::I>;
+    fn gen_copy_reg_to_retval(&self, idx: usize, from_reg: Writable<Reg>) -> Vec<Self::I>;
 
     /// Generate a return instruction.
     fn gen_ret(&self) -> Self::I;