RFR: 8320353: Reenable stringop-overflow warnings

[Anton Artemov]

Hi, please consider the following changes:

this PR addresses the issue of stringop-overflow warnings produced by GCC. The compiler does think that the thread pointer returned by `JavaThread::current()` can be null, though it cant. The thread pointer ends up being an argument in `__atomic_load`, and the compiler reports the warning related to argument of that method. 

The patch adds a hint to the compiler by means of `__builtin_unreachable()` intrinsic, which tells the compiler that certain piece of code will never be reached (case of thread pointer being null). This solves the issue. 

Tested in tiers 1-3 and GHA.

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Commit messages:
 - 8320353: Fixed comment
 - 8320353: Fixed whitespace error
 - 8320353:  Fiixed build problem
 - 8320353: Added debug only case
 - 8320353: More work
 - 8320353: More work
 - 8320353: Reenabled stringop-overflow warning for linux zero/fastdebug

Changes: https://git.openjdk.org/jdk/pull/26067/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26067&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320353
  Stats: 25 lines in 3 files changed: 20 ins; 4 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/26067.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26067/head:pull/26067

PR: https://git.openjdk.org/jdk/pull/26067