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

configure.ac

@@ -186,12 +186,6 @@ PGAC_ARG_BOOL(enable, rpath, yes,
               [do not embed shared library search path in executables])
 AC_SUBST(enable_rpath)
 
-#
-# Atomic operations
-#
-PGAC_ARG_BOOL(enable, atomics, yes,
-              [do not use atomic operations])
-
 #
 # --enable-debug adds -g to compiler flags
 #
@@ -1290,13 +1284,6 @@ failure.  It is possible the compiler isn't looking in the proper directory.
 Use --without-zlib to disable zlib support.])])
 fi
 
-if test "$enable_atomics" = yes; then
-  AC_DEFINE(HAVE_ATOMICS, 1, [Define to 1 if you want to use atomics if available.])
-else
-  AC_MSG_WARN([
-*** Not using atomic operations will cause poor performance.])
-fi
-
 if test "$with_gssapi" = yes ; then
   if test "$PORTNAME" != "win32"; then
     AC_SEARCH_LIBS(gss_store_cred_into, [gssapi_krb5 gss 'gssapi -lkrb5 -lcrypto'], [],