feat(build): add CONFIG_COMPILER_CXX_ATOMIC_LOCK_POLICY option

libstdc++ selects between a lock-free and a mutex-based policy for
std::shared_ptr reference counting based on this macro. Enabling this
option forces the lock-free policy, which avoids pulling in the
mutex-based implementation and can reduce code size.

Note: the lock-free policy avoids embedding a mutex in each control
block, saving about 88 bytes per shared_ptr, and improves performance
by using atomic operations instead of mutex locking.

Note: this option changes the ABI of libstdc++ atomic/shared_ptr
helpers. It can be incompatible when linking against prebuilt
libraries that were compiled without it, leading to link errors or
undefined runtime behavior. Only enable it if all C++ objects and
libraries in the build use the same setting.

tools/test_build_system/test_cxx_atomic_policy.py

@@ -0,0 +1,25 @@
+# SPDX-FileCopyrightText: 2026 Espressif Systems (Shanghai) CO LTD
+# SPDX-License-Identifier: Apache-2.0
+import pytest
+from test_build_system_helpers import IdfPyFunc
+
+CXX_ATOMIC_POLICY_TEST_APP = 'components/cxx/test_apps/atomic_lock_policy'
+
+
+@pytest.mark.test_app_copy(CXX_ATOMIC_POLICY_TEST_APP)
+@pytest.mark.usefixtures('test_app_copy')
+def test_cxx_atomic_policy_checking(idf_py: IdfPyFunc) -> None:
+    """
+    Build the atomic_lock_policy test app with -DTEST_INVALID_LOCK_POLICIES=1.
+
+    CONFIG_COMPILER_CXX_ATOMIC_LOCK_POLICY adds -D_GLIBCXX_HAVE_ATOMIC_LOCK_POLICY=1
+    to all C++ files, but TEST_INVALID_LOCK_POLICIES undefines it for one
+    translation unit. The linked executable therefore contains mixed
+    __gnu_cxx::_Lock_policy instantiations, and the cxx post-build hook
+    (check_cxx_atomic_policy.cmake) must fail the build.
+    """
+    ret = idf_py('-DIDF_TARGET=esp32c5', '-DTEST_INVALID_LOCK_POLICIES=1', 'build', check=False)
+    assert ret.returncode != 0, 'Build must fail when atomic lock policies are mixed'
+    assert 'Mixed libstdc++ atomic lock policies detected' in (ret.stdout + ret.stderr), (
+        'Expected the cxx post-build hook to report mixed atomic lock policies'
+    )