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

[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.

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#### 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.

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#### 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.

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Commit messages:
 - Clear inline caches before calling post_init
 - Fix relocations before registering nmethod
 - Add is_unloading() check before aquiring ic lock

Changes: https://git.openjdk.org/jdk/pull/28241/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28241&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371046
  Stats: 53 lines in 6 files changed: 28 ins; 21 del; 4 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