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

[Ioi Lam]

On Tue, 28 Jun 2022 07:20:59 GMT, David Holmes <dholmes at openjdk.org> wrote:

> Just took a quick skim through to get the general sense of things. Header file split is okay. Pity about the .cpp situation though - maybe move to platform_posix.cpp and platform_windows.cpp to at least get them out of the os_xxx.cpp file?

Unfortunately it's worse than I thought. There's a static function, `to_abstime()` that shared between `os::Posix::to_RTC_abstime()` and the `PlatformEvent::park()`.

To clean things up, I would need to move some static fields/functions into the `os::Posix` class. Maybe I should do the refactoring in a separate PR, to keep this PR simple?

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