RFR: 8277444: Race condition on Instrumentation.retransformClasses() and class linking [v3]

[Evgeny Astigeevich]

> There is a race between `JvmtiClassFileReconstituter::copy_bytecodes` and `InstanceKlass::link_class_impl`.  `InstanceKlass::link_class_impl` can be rewriting bytecodes. `JvmtiClassFileReconstituter::copy_bytecodes` will not restore them to the original ones because the flag `rewritten` is `false`. This will result in invalid bytecode.
> 
> This PR adds a lock (`init_lock`) to the `copy_bytecodes` method to prevent reading bytecodes while they are being rewritten during class linking.
> 
> Tested fastdebug and release builds: Linux x86_64 and arm64
> - The reproducer from JDK-8277444 passed.
> - Tier1 - tier3 passed.

Evgeny Astigeevich has updated the pull request incrementally with one additional commit since the last revision:

  Symplify comments; Get JavaThread::current in variable

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/26863/files
  - new: https://git.openjdk.org/jdk/pull/26863/files/d6895181..293d81ff

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26863&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26863&range=01-02

  Stats: 7 lines in 1 file changed: 0 ins; 1 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/26863.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26863/head:pull/26863

PR: https://git.openjdk.org/jdk/pull/26863