Update the manual page with new options

This pull request updates the rustc manual page to represent current state of rustc option handling better. Moved the apparently deprecated options (#19900) to their own section and added all the new codegen options.

A bit unrelatedly, I also updated description of `-O` and `-g` flags to point to the new codegen options rather than old, deprecated ones.

Fixes #20111.
Sorry about that, and thanks for this!