RFR: 8265473: Move os::Linux to its own header file

Fri Jul 8 06:27:56 UTC 2022

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.

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Commit messages:
 - 8265473: Move os::Linux to its own header file

Changes: https://git.openjdk.org/jdk/pull/9423/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9423&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8265473
  Stats: 431 lines in 12 files changed: 16 ins; 401 del; 14 mod
  Patch: https://git.openjdk.org/jdk/pull/9423.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9423/head:pull/9423

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