[libclc]: clspv: add a dummy implememtation for mul_hi

[rjodinchr]

clspv uses a better implementation that is not using a bigger side when not available.
Add a dummy implementation for mul_hi to avoid to override the implementation of clspv with the one in libclc.

[rjodinchr]

@frasercrmck could you please review?

[rjodinchr]

For the record, clspv implementation is based on OpMulExtended which is SPIR-V specific and not accessible from C.

[frasercrmck]

Seems alright to me.

It's a shame we've got such an ugly way of "opting out" of builtins, but I don't know if it's worth coming up with a nicer one.

[rjodinchr]

It's a shame we've got such an ugly way of "opting out" of builtins, but I don't know if it's worth coming up with a nicer one.

Yes, I don't think it's worth it right now.

Could you please submit that PR?

[frasercrmck]

It's a shame we've got such an ugly way of "opting out" of builtins, but I don't know if it's worth coming up with a nicer one.

Yes, I don't think it's worth it right now.

Could you please submit that PR?

Sure thing.

One thing I wondered is whether the empty file could be given a comment about why it's intentionally left empty. What do you think?