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
@@ -538,6 +538,10 @@ if test "$GCC" = yes -a "$ICC" = no; the
   if test -n "$NOT_THE_CFLAGS"; then
     CFLAGS="$CFLAGS -Wno-stringop-truncation"
   fi
+  if test x"$host_cpu" == x"aarch64"; then
+    PGAC_PROG_CC_CFLAGS_OPT([-moutline-atomics])
+    PGAC_PROG_CXX_CFLAGS_OPT([-moutline-atomics])
+  fi
 elif test "$ICC" = yes; then
   # Intel's compiler has a bug/misoptimization in checking for
   # division by NAN (NaN == 0), -mp1 fixes it, so add it to the CFLAGS.