Remove --disable-atomics, require 32 bit atomics.

Remove --disable-atomics, require 32 bit atomics.

Modern versions of all relevant architectures and tool chains have
atomics support.  Since edadeb07, there is no remaining reason to carry
code that simulates atomic flags and uint32 imperfectly with spinlocks.
64 bit atomics are still emulated with spinlocks, if needed, for now.

Any modern compiler capable of implementing C11 <stdatomic.h> must have
the underlying operations we need, though we don't require C11 yet.  We
detect certain compilers and architectures, so hypothetical new systems
might need adjustments here.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us> (concept, not the patch)
Reviewed-by: Andres Freund <andres@anarazel.de> (concept, not the patch)
Discussion: https://postgr.es/m/3351991.1697728588%40sss.pgh.pa.us