Omit non-needs_drop drop_in_place in vtables

This replaces the drop_in_place reference with null in vtables. On
librustc_driver.so, this drops about ~17k dynamic relocations from the
output, since many vtables can now be placed in read-only memory, rather
than having a relocated pointer included.

This makes a tradeoff by adding a null check at vtable call sites.
That's hard to avoid without changing the vtable format (e.g., to use a
pc-relative relocation instead of an absolute address, and avoid the
dynamic relocation that way). But it seems likely that the check is
cheap at runtime.

compiler/rustc_codegen_cranelift/src/base.rs

@@ -548,10 +548,7 @@ fn codegen_fn_body(fx: &mut FunctionCx<'_, '_, '_>, start_block: Block) {
             }
             TerminatorKind::Drop { place, target, unwind: _, replace: _ } => {
                 let drop_place = codegen_place(fx, *place);
-                crate::abi::codegen_drop(fx, source_info, drop_place);
-
-                let target_block = fx.get_block(*target);
-                fx.bcx.ins().jump(target_block, &[]);
+                crate::abi::codegen_drop(fx, source_info, drop_place, *target);
             }
         };
     }