Use `jit_core_sources` from build_varliables.bzl (#38526)
Summary:
Replace hardcoded filelist in aten/src/ATen/CMakeLists.txt with one from `jit_source_sources`
Fix `append_filelist` to work independently from the location it was invoked
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38526
Differential Revision: D21594582
Pulled By: malfet
fbshipit-source-id: c7f216a460edd474a6258ba5ddafd4c4f59b02be
