Add a note to help with missing `lld` (#1144)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1144

This note was in the old doc at https://github.com/pytorch/executorch/blob/3a74a1f06ce13a1953b530b9066ee1fc3e75b563/docs/website/docs/tutorials/00_setting_up_executorch.md?plain=1#L139-L142 but didn't make it to the new doc.

Fixes https://github.com/pytorch/executorch/issues/1142

Reviewed By: JacobSzwejbka

Differential Revision: D50991455

fbshipit-source-id: a9855377b15c8e38739de59da4acae5e266709dc