RFR: 8289230: Move PlatformXXX class declarations out of os_xxx.hpp [v2]

[Ioi Lam]

On Tue, 28 Jun 2022 19:39:29 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> Ioi Lam has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   fixed comments
>
> I very much like this incremental approach to reducing our over-inclusion.

Thanks @coleenp @calvinccheung @dholmes-ora for the review.

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PR: https://git.openjdk.org/jdk/pull/9303