gh-116738: Fix thread-safety issue in re module for free threading (gh-141923)

Added atomic operations to `scanner_begin()` and `scanner_end()` to prevent
race conditions on the `executing` flag in free-threaded builds. Also added
tests for concurrent usage of the `re` module.

Without the atomic operations, `test_scanner_concurrent_access()` triggers
`assert(self->executing)` failures, or a thread sanitizer run emits errors.

Include/internal/pycore_pyatomic_ft_wrappers.h

@@ -77,6 +77,10 @@ extern "C" {
     _Py_atomic_store_ushort_relaxed(&value, new_value)
 #define FT_ATOMIC_LOAD_USHORT_RELAXED(value) \
     _Py_atomic_load_ushort_relaxed(&value)
+#define FT_ATOMIC_LOAD_INT(value) \
+    _Py_atomic_load_int(&value)
+#define FT_ATOMIC_STORE_INT(value, new_value) \
+    _Py_atomic_store_int(&value, new_value)
 #define FT_ATOMIC_STORE_INT_RELAXED(value, new_value) \
     _Py_atomic_store_int_relaxed(&value, new_value)
 #define FT_ATOMIC_LOAD_INT_RELAXED(value) \
@@ -144,6 +148,8 @@ extern "C" {
 #define FT_ATOMIC_STORE_SHORT_RELAXED(value, new_value) value = new_value
 #define FT_ATOMIC_LOAD_USHORT_RELAXED(value) value
 #define FT_ATOMIC_STORE_USHORT_RELAXED(value, new_value) value = new_value
+#define FT_ATOMIC_LOAD_INT(value) value
+#define FT_ATOMIC_STORE_INT(value, new_value) value = new_value
 #define FT_ATOMIC_LOAD_INT_RELAXED(value) value
 #define FT_ATOMIC_STORE_INT_RELAXED(value, new_value) value = new_value
 #define FT_ATOMIC_LOAD_UINT_RELAXED(value) value