gatomic: Fix Clang type qualifier warnings

The actual type likely has the “volatile” qualifier, which then ends up being used for the local variable which gets its address passed to __atomic_load() and __atomic_compare_exchange_n().

This results in warnings such as:

warning: passing 'typeof (*(&g_define_type_id__volatile)) *' (aka
'volatile unsigned long *') to parameter of type 'gsize *' (aka
'unsigned long *') discards qualifiers