[llvm] [CI] Enable Offload project in precommit CI (PR #109103)

[Louis Dionne via llvm-commits]

https://github.com/ldionne commented:

AFAIU this should be all that's needed to cause the `offload` target to build whenever something under `offload/` is modified.

The important part now is whether building with `monolithic-linux.sh` works for the offload project (ideally it would, since that's a pretty standard CMake invocation).

https://github.com/llvm/llvm-project/pull/109103