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

[Romaric Jodin via cfe-commits]

rjodinchr 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?
> 
> Hmm yes you might be right. Just to confirm, does that fix the problem for you?
> 
> I can't be sure but it seems like that was a problem before [61efea7](https://github.com/llvm/llvm-project/commit/61efea7142e904e6492e1ce0566ec23d9d221c1e) when I last touched it, as least from a simple reading of the CMake.

I think the issue is not coming from the DEPENDS but from the source.
```
- COMMAND ${LLVM_SPIRV} ${spvflags} -o "${spv_suffix}" ${builtins_opt_lib_tgt}
+ COMMAND ${LLVM_SPIRV} ${spvflags} -o "${spv_suffix}" ${builtins_link_lib_tgt}
```

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