RFR: 8289230: Move PlatformXXX class declarations out of os_xxx.hpp [v2]

[Ioi Lam]

> There are only two implementations of these classes (one for windows, and one for posix):
> 
> - PlatformEvent
> - PlatformParker
> - PlatformMutex
> - PlatformMonitor
> - ThreadCrashProtection
> 
> Before this PR, these classes are declared in os_xxx.hpp. This causes excessive inclusion of the large header file os.hpp by popular headers such as mutex.hpp, which needs only the declaration of PlatformMutex but not the other stuff in os.hpp
> 
> This PR moves the declarations to park_posix.hpp, mutex_posix.hpp, etc.
> 
> Note:  ideally, the definition of PlatformParker/PlatformEvent should be moved to park_posix.cpp, and PlatformMutex/PlatformMonitor should be moved to mutex_posix.cpp. However, the definition of these 4 classes are intertwined, so I'll leave them inside os_posix.cpp for now. (Same for the Windows version).

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

  fixed comments

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/9303/files
  - new: https://git.openjdk.org/jdk/pull/9303/files/27ddeb07..8ceda0f5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9303&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9303&range=00-01

  Stats: 3 lines in 3 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/9303.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9303/head:pull/9303

PR: https://git.openjdk.org/jdk/pull/9303