pythongh-134875: Fix mimallc build error for the old compilers (pythongh-134994)

Author: corona10

Include/internal/mimalloc/mimalloc/internal.h

@@ -634,10 +634,10 @@ static inline mi_block_t* mi_block_nextx( const void* null, const mi_block_t* bl
   mi_track_mem_defined(block,sizeof(mi_block_t));
   mi_block_t* next;
   #ifdef MI_ENCODE_FREELIST
-  next = (mi_block_t*)mi_ptr_decode(null, mi_atomic_load_relaxed(&block->next), keys);
+  next = (mi_block_t*)mi_ptr_decode(null, mi_atomic_load_relaxed((_Atomic(mi_encoded_t)*)&block->next), keys);
   #else
   MI_UNUSED(keys); MI_UNUSED(null);
-  next = (mi_block_t*)mi_atomic_load_relaxed(&block->next);
+  next = (mi_block_t*)mi_atomic_load_relaxed((_Atomic(mi_encoded_t)*)&block->next);
   #endif
   mi_track_mem_noaccess(block,sizeof(mi_block_t));
   return next;