[libclc] [cmake] Fix per-target *_convert.cl dependencies (#127315)
Fix `add_libclc_builtin_set` to add an appropriate dependency to either
`clspv-generate_convert.cl` or `generate_convert.cl` based on the `ARCH`
argument, rather than to both unconditionally. This fixes build failures
due to missing dependencies when `clspv*` targets are not enabled.
The added check mirrors the one from `libclc/CMakeLists.txt`.
Fixes: #127378