Use newer config.sub to fix build on Apple M1 (#6854)

Use newer config.sub to fix build on Apple M1 (#6854)

When building cvc5 on macOS for M1 natively, ANTLR's version of
`config.sub` does not recognize `aarch64-apple-darwin20.5.0`. This
commit fixes that issue by downloading the latest version of
`config.sub` (similar to what we are already doing for `config.guess`).
The commit also updates the URLs for `config.guess` and `config.sub` to
the URLs recommended in the files themselves. The commit also does some
minor cleanup/simplifications of the commands for building ANTLR.