[llvm] Fix use of `MLIR_SRC_SHARDER_TABLEGEN` when cross compiling (PR #124008)

[NAKAMURA Takumi via llvm-commits]

https://github.com/chapuni requested changes to this pull request.

I think this is a bad workaround.

Excuse me, I don't understand the structure of mlir. Isn't it exported (installed)?

https://github.com/llvm/llvm-project/pull/124008