Support selector for functions.yaml and custom_ops.yaml targets in et_operator_library (#5957)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/5957

executorch_generated_library accepts

        functions_yaml_target: A Buck target pointing to the `functions.yaml`
            file to use. Optional, but at least one of `functions_yaml_target`
            and `custom_ops_yaml_target` must be specified.

        custom_ops_yaml_target: A Buck target pointing to the `custom_ops.yaml`
            file to use. Optional, but at least one of `functions_yaml_target`
            and `custom_ops_yaml_target` must be specified.

However the rule expects these targets to be the string of the exact target "//xplat/..../functions_yaml:functions.yaml"
and had issues with the parameter being of type 'selector'
select({"//arvr:my_constraint": "//xplat/..../functions_yaml:functions.yaml", ....})

Selectively providing this target based off of config/constraint can be helpful, and it seems like the limitation was the formatting done on the strings prior to calling genrules inside of the rule.

What this enables:
Building the same et_operator_library target while specifying the functions.yaml and custom_ops.yaml targets as a function of a constraint, like processor type.

Example: D64004596

Reviewed By: tarun292, larryliu0820

Differential Revision: D63043331

fbshipit-source-id: 9ef00f2a7625b0b29ae860513d1044923d0a4a34