RFR: 8265473: Move os::Linux to its own header file [v3]

[Ioi Lam]

On Fri, 8 Jul 2022 18:16:35 GMT, Ioi Lam <iklam at openjdk.org> wrote:

>> Another step of moving unnecessary stuff outside of os.hpp
>> 
>> The `os::Linux` class is used only by the Linux-specific code in HotSpot. Therefore, it should be moved outside of os.hpp, which is used by platform-independent code.
>> 
>> I don't have a good name for the new header. `os_linux.hpp` would have been a good name, but that's already taken, so I am settling on os_linux.impl.hpp. Suggestions are welcome.
>
> Ioi Lam has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fixed other linux variants

Another place that the os::Linux file could be moved to is [os_share_linux.hpp](https://github.com/openjdk/jdk/blob/master/src/hotspot/os/linux/os_share_linux.hpp). Today it contains some outdated declarations that are not used by anyone. 

If we interpret the name of this header to be "shared interfaces used by the os/*.cpp files", then os::Linux would belong here.

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PR: https://git.openjdk.org/jdk/pull/9423