action: support ATOMIC_INIT

action: support ATOMIC_INIT

For now, atomic_init() will be supported as simply a relaxed write. This
doesn't quite match the spec exactly [1, 2], but it is expedient for supporting
the common case of checking proper initialization.

[1] From C++ N3242, Section 29.6.5, Statement 8:

       Non-atomically initializes *object with value desired.

       Note: Concurrent access from another thread, even via an atomic
       operation, constitutes a data race.

[2] Note that for now, my implementation will not flag concurrent atomic_init()
    and atomic_store() as data races.