Resolve mlir_runner_utils on Darwin via .dylib extension

[FedericoBruzzone]

After running uv run lit . after #267 one test still fails:

Failed Tests (1):
  Lighthouse test suite :: test/run/pipeline-check.mlir

The problem is relatively simple to fix. Currently, lighthouse assumes a .so extension for shared libs. Unfortunately, on macOS the extension is .dylib. This PR introduces the _SHARED_EXT var to handle it properly.

Any feedback is welcome :D

Stacked on: #267

[FedericoBruzzone]

P.S. I'll rebase as soon as #267 is merged to make the review process smooth. However, the new changes are isolated in 91f515d .

@rengolin @adam-smnk

[rengolin]

Can you just merge this with the previous PR? Should be a trivial change.

[FedericoBruzzone]

Can you just merge this with the previous PR? Should be a trivial change.

Sure, thank you. I'll update the other PR description :D