sys/linux, sys/freebsd: apply more ignore_return attributes

1. Apply ignore_return to semctl$GETVAL which produces random errno
values on linux and freebsd.

2. Apply ignore_return to prctl and remove the custom code in executor.

3. Remove the custom errno ignoring code in fuchsia executor.
The calls are already marked as ignore_return, so this is just a leftover.

4. Only reset errno for ignore_return.
The syscall can still return a resource (maybe).
We only need to reset errno for fallback coverage.

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