Link against libatomic for 64-bit atomic operations

Some platforms (e.g., armel) require linking against libatomic for
64-bit atomic operations.

Fixes #20549

m4/fp_cc_supports__atomics.m4

@@ -8,27 +8,53 @@ AC_DEFUN([FP_CC_SUPPORTS__ATOMICS],
     AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[int x, y;]], [[__atomic_load(&x, &y, __ATOMIC_SEQ_CST); return y]])],
     [
         AC_MSG_RESULT(yes)
+
+        need_latomic=0
         AC_MSG_CHECKING(whether -latomic is needed for sub-word-sized atomic operations)
         AC_LINK_IFELSE([AC_LANG_PROGRAM([[unsigned char a;]], [[__atomic_fetch_or(&a, 1, __ATOMIC_RELAXED);]])],
         [
             AC_MSG_RESULT(no)
-            AC_SUBST([CabalNeedLibatomic],[False])
-            need_latomic=0
         ],
         [
-            _save_LIBS=$LIBS
+            _save_LIBS="$LIBS"
             LIBS="-latomic"
             AC_LINK_IFELSE([AC_LANG_PROGRAM([[unsigned char a;]], [[__atomic_fetch_or(&a, 1, __ATOMIC_RELAXED);]])],
             [
                 AC_MSG_RESULT(yes)
-                AC_SUBST([CabalNeedLibatomic],[True])
                 need_latomic=1
             ],
             [
                 AC_MSG_RESULT(failed)
                 AC_MSG_ERROR([sub-word-sized atomic operations are not available.])
             ])
-            LIBS=$_save_LIBS
+            LIBS="$_save_LIBS"
+        ])
+        AC_MSG_CHECKING(whether -latomic is needed for 64-bit atomic operations)
+        AC_LINK_IFELSE([AC_LANG_PROGRAM(
+            [[
+            #include <inttypes.h>
+            uint64_t a;
+            ]], [[__atomic_fetch_or(&a, 1, __ATOMIC_RELAXED);]])],
+        [
+            AC_MSG_RESULT(no)
+        ],
+        [
+            _save_LIBS="$LIBS"
+            LIBS="-latomic"
+            AC_LINK_IFELSE([AC_LANG_PROGRAM(
+                [[
+                #include <inttypes.h>
+                uint64_t a;
+                ]], [[__atomic_fetch_or(&a, 1, __ATOMIC_RELAXED);]])],
+            [
+                AC_MSG_RESULT(yes)
+                need_latomic=1
+            ],
+            [
+                AC_MSG_RESULT(failed)
+                AC_MSG_ERROR([64-bit atomic operations are not available.])
+            ])
+            LIBS="$_save_LIBS"
         ])
     ],
     [
@@ -36,6 +62,11 @@ AC_DEFUN([FP_CC_SUPPORTS__ATOMICS],
         AC_MSG_ERROR([C compiler needs to support __atomic primitives.])
     ])
     AC_DEFINE([HAVE_C11_ATOMICS], [1], [Does C compiler support __atomic primitives?])
+    if test "$need_latomic" = 1; then
+        AC_SUBST([CabalNeedLibatomic],[True])
+    else
+        AC_SUBST([CabalNeedLibatomic],[False])
+    fi
     AC_DEFINE_UNQUOTED([NEED_ATOMIC_LIB], [$need_latomic],
-        [Define to 1 if we need -latomic for sub-word atomic operations.])
+        [Define to 1 if we need -latomic.])
 ])