cbuilder: adapt switch generation with if fallbacks

[metagn]

Follows up #24420, uses a similar method of using untyped params with injected variables as #24410 to replace format string arguments.

[github-actions]

Thanks for your hard work on this PR!
The lines below are statistics of the Nim compiler built from ccb72c7

Hint: mm: orc; opt: speed; options: -d:release
177464 lines; 8.896s; 652.324MiB peakmem