Add support for C macro for_each_online_cpu

[Richardhongyu]

Currently, rust-for-linux does not have support for the for_each_online_cpu C macro. It is necessary for some drivers which need to know about the online cpus during the working phase.

It's hard to implement a function that has the same ability as this macro. Because this macro wraps some statements inside rather than acts as a part of some statements. I ask for advice in the Zulip. Wedson suggests we should implement an iterator that returns all the cpu indices.

I give a try to implement an iterator for for_each_online_cpu. It works fine in my local test. I want to ask for some advice and see whether it can be merged.

[Richardhongyu]

The corresponding implementation is in #728. There are still other for_each_* macros that I don't implement for now. But the basic idea is similar. I want to ask if this PR is appropriate before getting all the code done.