ci: debug builds are broken with change to x86 backend

[travisstaloch]

https://github.com/travisstaloch/simdjzon/actions/runs/15524140371/job/43701302519

error: invalid constraint: '+x'

Upstream issue: ziglang/zig#24046

Resolve by either

• waiting for the issue to be resolved
• use llvm backend for debug builds

[travisstaloch]

I've add some exe.use_llvm = true to the build script to get tests passing for now.