[libclc] [libclc]: clspv: add a dummy implememtation for mul_hi (PR #134094)

[Romaric Jodin via cfe-commits]

rjodinchr wrote:

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

https://github.com/llvm/llvm-project/pull/134094