Build and link against sentencepiece 3rd party lib. (#4410)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4410

bypass-github-export-checks
bypass-github-pytorch-ci-checks
bypass-github-executorch-ci-checks

Reviewed By: kirklandsign

Differential Revision: D60210064

fbshipit-source-id: 675a2f9c91f972cc080c820f2cafd8ec6f7c9401