atomics: add SC_ATOMIC_INITPTR macro

atomics: add SC_ATOMIC_INITPTR macro

Until now both atomic ints and pointers were initialized by SC_ATOMIC_INIT
by setting them to 0. However, C11's atomic pointer type cannot be
initialized this way w/o causing compiler warnings.

As a preparation to supporting C11's atomics, this patch introduces a
new macro to initialize atomic pointers and updates the relevant callers
to use it.