RFR: 8290840: Refactor the "os" class [v7]

[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.

Ioi Lam has updated the pull request incrementally with one additional commit since the last revision:

  @stefank comments

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/9600/files
  - new: https://git.openjdk.org/jdk/pull/9600/files/b6e60a54..11c3c3d3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9600&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9600&range=05-06

  Stats: 24 lines in 14 files changed: 11 ins; 13 del; 0 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