build: add option to choose to regenerate cmd_descs.[ch]

[ret2libc]

Your checklist for this pull request

• I've read the guidelines for contributing to this repository
• I made sure to follow the project's coding style
• I've documented or updated the documentation of every function and struct this PR changes. If not so I've explained why.
• I've added tests that prove my fix is effective or that my feature works (if possible)
• I've updated the rizin book with the relevant information (if needed)

Detailed description

• 'auto' => if pyyaml is available, cmd_descs are regenerated, otherwise they are not.
• 'enabled' => if pyyaml is not available, it fails. if it is available, cmd_descs are regenerated
• 'disabled' => cmd_descs are not regenerated

Test plan

• meson -Dregenerate_cmds=auto build1 on a system with/without pyyaml
• meson -Dregenerate_cmds=enabled build2 on a system with/without pyyaml
• meson -Dregenerate_cmds=disabled build3

Closing issues

Closes #2926