Support correct charset for @-files

[gnodet]

The d2dd532 brought an inconsistency that I saw with a project containing chinese characters and I made it consistent with 33c9f01

I assume that this bug comes from something related to the way we are determining the charset in that getDataCharset function. It might be needed to make the decision making depending not only on java version but also on os/arch.

I will test this fix. If it is not making the previous bug come back, we can just leave it as it is, but if it regresses, we should look for a proper fix.

@slachiewicz have a look at https://issues.apache.org/jira/browse/MJAVADOC-614, it mentions the intent for the JDK check, and the fact that the @-files is using UTF-8 on JDK 9..12 and Charset.defaultSet() on others.

Originally posted by @gnodet in #1274 (comment)