specify their own version of atomic_cmpset_* which could have been
different than the membar version.
Right now, however, FreeBSD is bound mostly to GCC-like compilers and
it is desired to add new support and compat shim mostly when there is
a real necessity, in order to avoid too much compatibility bloats.
In this optic, bring back atomic_cmpset_{acq, rel}_* to be the same as
atomic_cmpset_* and unwind the atomic_cmpset_barr_* introduction.
Requested by: jhb
Reviewed by: jhb
Tested by: Giovanni Trematerra <giovanni dot trematerra at
gmail dot com>
8448afced8