always use _Atomic() type specifier for non standard atomics

...instead of the _Atomic type qualifier.

This allows tweaking the type when stdatomic.h is not supported as it turns into the declaration into a macro.

Restrictions on the type specifier (now): The type name in an atomic type specifier shall not refer to an array type, a function type, an atomic type, or a qualified type.

Restrictions on the type qualifier (before): The type modified by the _Atomic qualifier shall not be an array type or a function type.

So it cannot apply on an atomic type (duh!) and a qualified type, so with the const, restrict, volatile qualifier.