While testing BIND 9 on arm64 8+ core machine, it was discovered that
the weak variants in fact does spuriously fail, we haven't observed that
on other architectures.
This commit replaces all non-loop usage of atomic_compare_exchange_weak
with atomic_compare_exchange_strong.
isc_event_allocate() calls isc_mem_get() to allocate the event structure. As
isc_mem_get() cannot fail softly (e.g. it never returns NULL), the
isc_event_allocate() cannot return NULL, hence we remove the (ret == NULL)
handling blocks using the semantic patch from the previous commit.
The differences between two files are very minimal and most of the
code is common. Merge those two files and use #ifdef WIN32 to include
the right bits on Windows.