RFR: 8265473: Move os::Linux to its own header file [v2]

[Ioi Lam]

> 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 gtest

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/9423/files
  - new: https://git.openjdk.org/jdk/pull/9423/files/8ab57889..c8bd68b4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9423&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9423&range=00-01

  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 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