RFR: 8260306: Do not include osThread.hpp in thread.hpp

[Ioi Lam]

thread.hpp is a popular header file (included by 856 out of 1000 HotSpot.o file), so we should try to minimize what files are included by thread.hpp.

We can replace the inclusion of osThread.hpp with a forward declaration of the OSThread class.

This reduces the number of .o files that include osThread.hpp to 42.

Tested with mach5: tier1, builds-tier2, builds-tier3, builds-tier4 and builds-tier5. Also locally: aarch64, arm, ppc64, s390, x86, and zero.

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Commit messages:
 - 8260306: Do not include osThread.hpp in thread.hpp

Changes: https://git.openjdk.java.net/jdk/pull/2198/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2198&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260306
  Stats: 31 lines in 19 files changed: 19 ins; 1 del; 11 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2198.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2198/head:pull/2198

PR: https://git.openjdk.java.net/jdk/pull/2198