BUILD: atomic: prefer __atomic_compare_exchange_n() for __ha_cas_dw()

BUILD: atomic: prefer __atomic_compare_exchange_n() for __ha_cas_dw()

__atomic_compare_exchange() is incorrectly documented in the gcc builtins
doc, it says the desired value is "type *desired" while in reality it is
"const type *desired" as expected since that value must in no way be
modified by the operation. However it seems that clang has implemented
it as documented, and reports build warnings when fed a const.

This is quite problematic because it means we have to betry the callers,
pretending we won't touch their constants but not knowing what the
compiler would do with them, and possibly hiding future bugs.

Instead of forcing a cast, let's just switch to the better
__atomic_compare_exchange_n() that takes a value instead of a pointer.
At least with this one there is no doubt about how the input will be
used.

It was verified that the output object code is the same both in clang
and gcc with this change.