LibC+UE: Keep more unused chunked blocks around

Previously each malloc size class would keep around a limited number of
unused blocks which were marked with MADV_SET_VOLATILE which could then
be reinitialized when additional blocks were needed.

This changes malloc() so that it also keeps around a number of blocks
without marking them with MADV_SET_VOLATILE. I termed these "hot"
blocks whereas blocks which were marked as MADV_SET_VOLATILE are called
"cold" blocks because they're more expensive to reinitialize.

In the worst case this could increase memory usage per process by
1MB when a program requests a bunch of memory and frees all of it.

Also, in order to make more efficient use of these unused blocks
they're now shared between size classes.

Userland/DevTools/UserspaceEmulator/Emulator_syscalls.cpp

@@ -1121,6 +1121,9 @@ int Emulator::virt$emuctl(FlatPtr arg1, FlatPtr arg2, FlatPtr arg3)
     case 3:
         tracer->target_did_realloc({}, arg3, arg2);
         return 0;
+    case 4:
+        tracer->target_did_change_chunk_size({}, arg3, arg2);
+        return 0;
     default:
         return -EINVAL;
     }