Remove usage of deprecated ATOMIC_VAR_INIT() macro
The C17 standard deprecated ATOMIC_VAR_INIT() macro (see [1]). Follow the suite and remove the ATOMIC_VAR_INIT() usage in favor of simple assignment of the value as this is what all supported stdatomic.h implementations do anyway:
- MacOSX.plaform: #define ATOMIC_VAR_INIT(__v) {__v}
- Gcc stdatomic.h: #define ATOMIC_VAR_INIT(VALUE) (VALUE)