Using these was a temporary solution while some compilers implemented
the underlying atomic mechanisms, but not the C11 language parts (or
that's what I guess). Not really useful for us anymore. Also, there is
the slight risk of having subtly incorrect semantics by using
potentially changing compiler internals and such.
The correctness of the stdatomic.h emulation via the __sync builtins is
questionable, and we've been relying on exact stdatomic semantics for a
while, so just get rid of it. Compilers which support __sync but not
stdatomic.h will use to the slow mutex fallback.
Not sure about the __atomic builtins. It doesn't seem to harm either, so
leave it for now.
