Enable outline-atomics on arm64.

The flag was added in focal's gcc, but is off by default there. It is enabled
by default on all later distributions (hirsute, impish, bullseye, bookwork,
sid).

https://www.postgresql.org/message-id/flat/1635221042457.21654%40amazon.com

This patch can be removed once focal is EOL.

--- a/configure.ac
+++ b/configure.ac
@@ -557,6 +557,10 @@
   if test -n "$NOT_THE_CFLAGS"; then
     CFLAGS="$CFLAGS -Wno-stringop-truncation"
   fi
+  if test x"$host_cpu" == x"aarch64"; then
+    PGAC_PROG_CC_CFLAGS_OPT([-moutline-atomics])
+    PGAC_PROG_CXX_CFLAGS_OPT([-moutline-atomics])
+  fi
   # Suppress clang 16's strict warnings about function casts
   NOT_THE_CFLAGS=""
   PGAC_PROG_CC_VAR_OPT(NOT_THE_CFLAGS, [-Wcast-function-type-strict])
