RFR: 8260306: Do not include osThread.hpp in thread.hpp [v2]

[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.

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 four additional commits since the last revision:

 - Fixed merge
 - Merge branch 'master' into 8260306-dont-include-osThread.hpp-in-thread.hpp
 - Merge branch 'master' of https://github.com/openjdk/jdk into 8260306-dont-include-osThread.hpp-in-thread.hpp
 - 8260306: Do not include osThread.hpp in thread.hpp

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2198/files
  - new: https://git.openjdk.java.net/jdk/pull/2198/files/4da3fecb..d6f22383

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2198&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2198&range=00-01

  Stats: 4945 lines in 137 files changed: 2573 ins; 1855 del; 517 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