[enhancement] Improve codegen_main default flag values

[grebe]

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.