RFR: 8291945: Add OSInfo API for static OS information [v3]

[Ioi Lam]

> As suggested by @tstuefe in the review of #9600, many of the `os` APIs simply return a value that was initialized during VM start up. E.g., `os::vm_page_size()`. I have added a new class `OSInfo` for managing such info
> 
> - Simplifies the per-OS implementation
> - Make the "init-only-once" check consistent
> - In some cases, the simpler `OSInfo` class can be used in lieu of the more complex `os` class. This improves HotSpot build time (e.g., relocInfo.hpp).
> 
> Note: I moved only 2 fields in this PR as a first step. If this works well I plan to move more fields in the future.

Ioi Lam has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:

 - Merge branch 'master' of https://github.com/openjdk/jdk into 8291945-add-OSInfo-api
 - @tstuefe review -- clarify comments
 - fixed typo
 - fixed zero build; added comments
 - 8291945: Add OSInfo API for static OS information

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/9758/files
  - new: https://git.openjdk.org/jdk/pull/9758/files/3da77fcb..faa28f65

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9758&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9758&range=01-02

  Stats: 7721 lines in 347 files changed: 4361 ins; 2178 del; 1182 mod
  Patch: https://git.openjdk.org/jdk/pull/9758.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9758/head:pull/9758

PR: https://git.openjdk.org/jdk/pull/9758