x64 and aarch64: allow StructArgument and StructReturn args.

The StructReturn ABI is fairly simple at the codegen/isel level: we only
need to take care to return the sret pointer as one of the return values
if that wasn't specified in the initial function signature.

Struct arguments are a little more complex. A struct argument is stored
as a chunk of memory in the stack-args space. However, the CLIF
semantics are slightly special: on the caller side, the parameter passed
in is a pointer to an arbitrary memory block, and we must memcpy this
data to the on-stack struct-argument; and on the callee side, we provide
a pointer to the passed-in struct-argument as the CLIF block param
value.

This is necessary to support various ABIs other than Wasm, such as that
of Rust (with the cg_clif codegen backend).

cranelift/codegen/src/isa/aarch64/lower.rs

@@ -1231,7 +1231,7 @@ impl LowerBackend for AArch64Backend {
     type MInst = Inst;
 
     fn lower<C: LowerCtx<I = Inst>>(&self, ctx: &mut C, ir_inst: IRInst) -> CodegenResult<()> {
-        lower_inst::lower_insn_to_regs(ctx, ir_inst)
+        lower_inst::lower_insn_to_regs(ctx, ir_inst, &self.flags)
     }
 
     fn lower_branch_group<C: LowerCtx<I = Inst>>(