[jdk25u] RFR: 8358735: GenShen: block_start() may be incorrect after class unloading

[Rui Li]

Backport of JDK-8358735.

Backport is almost clean except that `atomichpp` was renamed to `atomicAccess.hpp`.

Tier1 - Tier3 passed.

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Commit messages:
 - Backport 8531fa146be1da5e96c0f23091882a27c67d7893

Changes: https://git.openjdk.org/jdk25u/pull/378/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk25u&pr=378&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358735
  Stats: 893 lines in 12 files changed: 851 ins; 10 del; 32 mod
  Patch: https://git.openjdk.org/jdk25u/pull/378.diff
  Fetch: git fetch https://git.openjdk.org/jdk25u.git pull/378/head:pull/378

PR: https://git.openjdk.org/jdk25u/pull/378