GH-49272: [C++][CI] Fix intermittent segfault in arrow-json-test on M…

[vanshaj2023]

Rationale for this change

The arrow-json-test intermittently segfaults on AMD64 Windows MinGW CI (both CLANG64 and MINGW64 environments), causing false CI failures. The crash occurs at 0.03-0.07 seconds into test execution during the first parallel test (ReaderTest.MultipleChunksParallel). See #49272.

The root cause is MinGW's __emutls implementation for C++ thread_local, which has known race conditions during thread creation. When ThreadPool::LaunchWorkersUnlocked creates a new worker thread, the thread immediately writes to the thread_local ThreadPool* current_thread_pool_ variable. If __emutls hasn't finished initializing TLS for the new thread, this dereferences an invalid pointer, causing a segfault.

What changes are included in this PR?

1. Replace thread_local with native Win32 TLS API on MinGW (thread_pool.cc): Uses TlsAlloc/TlsGetValue/TlsSetValue instead of thread_local on MinGW to bypass the buggy __emutls emulation. Non-MinGW platforms are unchanged.

2. Strengthen atomic memory ordering in ThreadedTaskGroup (task_group.cc): Changed nremaining_ operations from memory_order_acquire/memory_order_release to memory_order_acq_rel as defensive hardening. This is zero-cost on x86.

3. Add SEH crash handler for MinGW test builds (reader_test.cc): Logs the exception code and address on crash, providing better debugging info if segfaults recur.

4. Add MultipleChunksParallelStress test (reader_test.cc): Runs parallel JSON reading 20 times to help expose intermittent threading races.

Are these changes tested?

Yes. A new stress test (ReaderTest.MultipleChunksParallelStress) is added that repeatedly exercises the parallel JSON reading path. The existing ReaderTest.MultipleChunksParallel and AsyncStreamingReaderTest.AsyncReentrancy tests also cover the affected code paths.

Are there any user-facing changes?

No.

This PR contains a "Critical Fix". The changes fix a bug that causes a crash (segfault) in arrow-json-test on MinGW Windows CI due to a race condition in MinGW's __emutls thread-local storage emulation during thread pool worker creation.

• GitHub Issue: [C++][CI] arrow-json-test segfaults occasionally on AMD64 Windows MinGW MINGW64 C++ job #49272