RFR: 8289710: Move Suspend/Resume classes out of os.hpp

[Ioi Lam]

Please review this simple change that only renames a few classes and moved some code around. No functional changes.

The following classes are used only sparingly. They should be moved to a new header file share/runtime/suspend.hpp to minimize the size of os.hpp

- SuspendedThreadTaskContext
- SuspendedThreadTask
- SuspendResume

I didn't move the OS-specific implementation to a new file -- the POSIX implementation is currently inside [signals_posix.cpp](https://github.com/openjdk/jdk/blob/df063f7db18a40ea7325fe608b3206a6dff812c1/src/hotspot/os/posix/signals_posix.cpp#L1790) mixed with other signal handling code, so it doesn't seem a good idea to move out just the code for the above 3 classes.

The only other implementation is in os_windows.cpp. I could move the code to suspend_windows.cpp, but I don't feel very motivated unless someone insists.

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Commit messages:
 - 8289710: Move Suspend/Resume classes out of os.hpp

Changes: https://git.openjdk.org/jdk/pull/9371/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9371&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289710
  Stats: 322 lines in 12 files changed: 172 ins; 114 del; 36 mod
  Patch: https://git.openjdk.org/jdk/pull/9371.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9371/head:pull/9371

PR: https://git.openjdk.org/jdk/pull/9371