RFR: 8290840: Refactor the "os" class [v2]

[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 two additional commits since the last revision:

 - moved os::{print_active_locale, print_user_info} to os_posix.cpp
 - Fixed os.hpp comments per @dholmes-ora review

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/9600/files
  - new: https://git.openjdk.org/jdk/pull/9600/files/5516640c..9f853295

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9600&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9600&range=00-01

  Stats: 52 lines in 6 files changed: 3 ins; 31 del; 18 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