Sync toplevel configure fixup

Apparently I missed that we needed to sync config/acx.m4. I only
noticed this because our packaging has a grep for certain messages
post-merge.

```
work/binutils/configure: 5814: ACX_PROG_CARGO: not found
```

Fix that by syncing the macro too, which I missed in 987db70acefd0b223a8df2240d4e5ca544cc0a91.