Remove usage of deprecated ATOMIC_VAR_INIT() macro

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)

1. http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1138r0.pdf

(cherry picked from commit f251d69ebad6f17190c890829897a2bc8f7d6783)