Split out :executor_runner_lib (#93)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/93

It will be helpful to create other runner targets that link against specific backends or kernel libs. Rather than adding those dependencies to this common target, create a library that can be used to create, e.g., `xnnpack_runner`.

Reviewed By: mcr229, cccclai

Differential Revision: D48579335

fbshipit-source-id: 95672a270e5c3880adb1effc32e04cb512d31eeb