Trying out <stdatomic.h>

Hi,

Here is an experimental patch to try out standard C (and C++) atomics
to implement port/atomics.h, and also add more types and operations.
It's mostly just redirecting our names to the standard ones, except
for our barriers and slightly extended pg_atomic_flag, so there isn't
much left of the code.

Here also is a semi-independent patch to implement storage/spin.h with
pg_atomic_flag. It keeps a small amount of the architecture-specific
magic, but moves it out to src/port/spin_delay.h.

I'm not saying they're correct, performant or portable yet, that'll
require studying codegen and performance on a bunch of weird and
wonderful systems, but they at least passes basic testing on the usual
suspects and CI systems, except for Windows which needs a newer
toolchain so I haven't tried yet. It should hopefully just work™ on
VS 2022 (same version that provides <threads.h>, about which more
soon).

All that being the case, it's not far enough along to be a serious
proposal, but I imagine others will be looking into things like this
since we pulled the trigger on C11 and I figured I might as well share
a basic working patch set to avoid duplicated effort... Hopefully it
works well enough to kick the tyres and try to find the difficult
problems, test it out on weird systems, etc etc.