julia_gc: enable guard page hack for Julia >= 1.12, too (#5913)

My change to make it unnecessary there was reverted, so
it is wrong to disable our workaround there.

src/julia_gc.c

@@ -420,27 +420,27 @@ static inline void * align_ptr(void * p)
 static void FindLiveRangeReverse(PtrArray * arr, void * start, void * end)
 {
     // HACK: the following deals with stacks of 'negative size' exposed by
-    // Julia -- however, despite us having this code in here for a few years,
-    // I now think it may actually be due to a bug on the Julia side. See
-    // <https://github.com/JuliaLang/julia/pull/54639> for details.
+    // Julia -- this was due to a bug on the Julia side. It was finally fixed
+    // by <https://github.com/JuliaLang/julia/pull/54639>, which should
+    // hopefully appear in Julia 1.12.
     if (lt_ptr(end, start)) {
         SWAP(void *, start, end);
     }
-#if JULIA_VERSION_MAJOR == 1 && JULIA_VERSION_MINOR <= 11
     // adjust for Julia guard pages if necessary
-    // In Julia >= 1.12 this is no longer necessary thanks
-    // to <https://github.com/JuliaLang/julia/pull/54591>
-    // TODO: hopefully this actually also gets backported to 1.11.0
+    //
+    // For a time I hoped this wouldn't be necessary anymore in Julia >= 1.12
+    // due to <https://github.com/JuliaLang/julia/pull/54591>  but that PR was
+    // later reverted by <https://github.com/JuliaLang/julia/pull/55555>.
     //
     // unfortunately jl_guard_size is not exported; fortunately it
     // is the same in all Julia versions were we need it
     else {
-        void * new_start = (char *)start + (4096 * 8);
+        const size_t jl_guard_size = (4096 * 8);
+        void * new_start = (char *)start + jl_guard_size;
         if ((uintptr_t)new_start <= (uintptr_t)end) {
             start = new_start;
         }
     }
-#endif
     char * p = (char *)(align_ptr(start));
     char * q = (char *)end - sizeof(void *);
     while (!lt_ptr(q, p)) {