RFR: 8371046: Segfault in compiler/whitebox/StressNMethodRelocation.java with -XX:+UseZGC [v2]

[Chad Rakoczy]

> [JDK-8371046](https://bugs.openjdk.org/browse/JDK-8371046)
> 
> This pull request fixes two crashes (see below) and adds `InvalidationReason::RELOCATED` to better describe why an nmethod is marked not entrant during relocation.
> 
> ---
> 
> #### 1. Test Bug
> 
> It’s possible for an `nmethod` to be unloaded without its `_state` being explicitly set to `not_entrant`. Checking only `is_in_use()` isn’t sufficient, since the `nmethod` may already be in the process of unloading and therefore may not have a lock (as with ZGC, where `nmethods` are locked individually).
> 
> The fix adds an additional `is_unloading()` check in WhiteBox before acquiring the lock.
> 
> This issue was reproducible fairly consistently (every few runs) by executing `compiler/whitebox/StressNMethodRelocation.java` with  `-XX:+UseZGC -XX:ReservedCodeCacheSize=32m`
> 
> 
> After applying this patch, the original crash stopped occurring, though a more infrequent crash was still observed.
> 
> ---
> 
> #### 2. Implementation Bug
> 
> `nmethod::relocate` works by copying the instructions of an `nmethod` and then adjusting the call sites to account for new PC-relative offsets.
> 
> Previously, this fix-up happened *after* calling `post_init()`, which registers the `nmethod` and makes it visible to the GC. This introduced a race condition where the GC might attempt to resolve a call site before it had been fixed.
> 
> The fix ensures that all call sites are patched **before** the `nmethod` is registered.
> 
> In testing, the crash previously occurred roughly 60 times in 5,000 runs (~1.2%). With this patch, no crashes were observed in the same number of runs.

Chad Rakoczy 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 six additional commits since the last revision:

 - Merge remote-tracking branch 'origin/master' into JDK-8371046
 - Add comment
 - Add guarantee
 - Clear inline caches before calling post_init
 - Fix relocations before registering nmethod
 - Add is_unloading() check before aquiring ic lock

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/28241/files
  - new: https://git.openjdk.org/jdk/pull/28241/files/dac140bb..7160f835

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28241&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28241&range=00-01

  Stats: 149874 lines in 621 files changed: 103255 ins; 19601 del; 27018 mod
  Patch: https://git.openjdk.org/jdk/pull/28241.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28241/head:pull/28241

PR: https://git.openjdk.org/jdk/pull/28241