[PPC] Add missing vec_[signed|neg|sldw] definitions (#50640)
Summary:
Base on quickwritereader 's comment: https://github.com/pytorch/pytorch/issues/50439#issuecomment-760025933
Those builtins were added in gcc-8 or newer
Fixes https://github.com/pytorch/pytorch/issues/50439
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50640
Reviewed By: walterddr
Differential Revision: D25934384
Pulled By: malfet
fbshipit-source-id: b5dcfcf644ab92a78279c4dca5dbffbb8d8aae0c
