Add support for IBM EBCDIC encodings

[depau]

This PR adds support for encodings in the IBM EBCDIC family (mainframes).

I'm not sure how to update the .po files so I haven't done that. How do I take care of it?

[hltj]

Thanks.
It looks good!

While it also needs related test cases, I will deal with it later.

[depau]

Thank you! Let me know if there's anything you need me to change. I'll try to add some tests when I have some more time.

[hltj]

I had checked the wikipedia entries about EBCDIC encodings, and I found that there are more encodings including variants for different use. I will select the encodings to be supported first, and then confirm whether there is any difference between different versions of ICU.

In addition, all previously supported encodings are iso646 compatible, I will confirm whether there is any compatibility problem with supporting EBCDIC.