`platform_data` required executable targets

This came up in a [code review discussion](https://pigweed-review.git.corp.google.com/c/pigweed/pigweed/+/212431/comments/7596d784_de6e9433) for an (accidental) re-implementation of `platform_data`: the `target` in `platform_data` is required to be `executable = True`. I thought the reason it's called `platform_data` and not `platform_binary` is specifically because the target doesn't have to be created by a binary rule, but can be any kind of data!

It would be nice to relax this restriction.