stdatomic.h defines no atomic_float typedef. We can't just use _Atomic
unconditionally, because we support compilers without C11 atomics. So
just create a custom atomic_float typedef in the wrapper, which uses
_Atomic in the C11 code path.
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.
wm4