Zend: Remove zend_atomic.[ch] abstraction (#23927)

Author: henderkes

UPGRADING.INTERNALS

@@ -17,6 +17,8 @@ PHP 8.7 INTERNALS UPGRADE NOTES
 - Removed zend_execute_scripts(). Manually call zend_execute_script() in a loop
   instead.
 - Removed ZEND_STATIC_ASSERT(). Use C11 static_assert() directly instead.
+- Removed zend_atomic.h and the zend_atomic_* types and functions.
+  Use the standard C11 atomic_* types and functions instead.
 - The sapi_terminate_process() function was removed.
 - The sapi_force_http_10() function was removed.
 - The sapi_get_fd() function was removed.
@@ -33,8 +35,8 @@ PHP 8.7 INTERNALS UPGRADE NOTES
 ========================
 
 - Windows build system changes:
-  . C sources are now compiled as C17. MSVC builds require Visual Studio
-    2026 or later and enable /experimental:c11atomics.
+  . C sources are now compiled as C17 and require C11 atomics. MSVC builds require
+    Visual Studio 2026 or later and enable /experimental:c11atomics.
   . C++-only extension compiler flags can be added to CXXFLAGS_<EXTENSION>.
 
 - Unix build system changes: