This further simplifies behavior to match the "build vs host" decision
we did with `c_args` vs `build_c_args`. The rules are now simply:
- `native: true` affects `native: true` targets
- `native: false` affects `native: false` targets
- No native flag is the same as `native: false`
I like this because you don't even have to know what "build" and "host"
mean to understand how it works, and it doesn't depend on whether the
overall build is cross or not.
Fixes#4933
John Ericson