RFR: 8290840: Refactor the "os" class

[Ioi Lam]

Please see [JDK-8290840](https://bugs.openjdk.org/browse/JDK-8290840) for the detailed proposal.

The `os` class, declared in os.hpp, forms the major part of the HotSpot porting interface. Its structure has gradually deteriorated over the years as new ports are created and new APIs are added.

This RFE tries to address the following:

- Clearly specify where a porting API should be declared and defined among the various `os*.cpp` and `os*.hpp` files.
- Avoid the inappropriate inclusion of OS-specific APIs (such as the `os::Linux class`) by platform-independent source files.

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Commit messages:
 - fixed whitespaces
 - 8290840: Refactor the "os" class

Changes: https://git.openjdk.org/jdk/pull/9600/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9600&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290840
  Stats: 1348 lines in 95 files changed: 657 ins; 609 del; 82 mod
  Patch: https://git.openjdk.org/jdk/pull/9600.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9600/head:pull/9600

PR: https://git.openjdk.org/jdk/pull/9600