don't access void* out of alignment in refc GC to avoid UB (#21560)

(cherry picked from commit e8a70ff179)

lib/system/gc_common.nim

@@ -381,7 +381,6 @@ else:
             let regEnd = sp +% sizeof(registers)
             while sp <% regEnd:
               gcMark(gch, cast[PPointer](sp)[])
-              gcMark(gch, cast[PPointer](sp +% sizeof(pointer) div 2)[])
               sp = sp +% sizeof(pointer)
         # Make sure sp is word-aligned
         sp = sp and not (sizeof(pointer) - 1)