[enhancement] Improve codegen_main default flag values

### What's hard to do? (limit 100 words)

Currently, the default flag values will not produce a useable output for most (all?) procs. Some things may not have reasonable defaults, but many of them do (e.g. channel port suffixes).

### Current best alternative workaround (limit 100 words)

Big "default arguments" dictionaries in bazel, wrapper scripts, etc.

### Your view of the "best case XLS enhancement" (limit 100 words)

`./codegen_main file.opt.ir` produces a useful, usable output. Interactive use doesn't require copy-pasting a huge number of flags.