RFR: 8291945: Add OSInfo API for static OS information

[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.

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Commit messages:
 - fixed zero build; added comments
 - 8291945: Add OSInfo API for static OS information

Changes: https://git.openjdk.org/jdk/pull/9758/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9758&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291945
  Stats: 213 lines in 17 files changed: 108 ins; 68 del; 37 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