[libclc] [libclc] Fix a couple of issues preventing in-tree builds (PR #87505)

[Michał Górny via cfe-commits]

mgorny wrote:

```
      set( spv_suffix ${arch_suffix}.spv )
      add_custom_command( OUTPUT "${spv_suffix}"
        COMMAND ${LLVM_SPIRV} ${spvflags} -o "${spv_suffix}" ${builtins_opt_lib_tgt}
        DEPENDS ${builtins_link_lib_tgt} )
```

I think that ought to be `DEPENDS ${builtins_opt_lib_tgt}`. Do you need me to send a PR for that?

https://github.com/llvm/llvm-project/pull/87505