sys/mman.h: add MADV_COLD and MADV_PAGEOUT from linux v5.4

sys/mman.h: add MADV_COLD and MADV_PAGEOUT from linux v5.4

These were mainly introduced so android can optimize the memory usage
of unused apps.

MADV_COLD hints that the memory range is currently not needed (unlike
with MADV_FREE the content is not garbage, it needs to be swapped):

  linux commit 9c276cc65a58faf98be8e56962745ec99ab87636
  mm: introduce MADV_COLD

MADV_PAGEOUT hints that the memory range is not needed for a long time
so it can be reclaimed immediately independently of memory pressure
(unlike with MADV_DONTNEED the content is not garbage):

  linux commit 1a4e58cce84ee88129d5d49c064bd2852b481357
  mm: introduce MADV_PAGEOUT