RFR: 8320353: Reenable stringop-overflow warnings

[Magnus Ihse Bursie]

On Tue, 1 Jul 2025 12:25:04 GMT, Anton Artemov <duke at openjdk.org> wrote:

> 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.

Build changes look good. Someone else will have to review the hotspot changes.

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Marked as reviewed by ihse (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/26067#pullrequestreview-2976656855