sys/mman.h: add MREMAP_DONTUNMAP from linux v5.7

it remaps anon mappings without unmapping the original. chromeos plans
to use it with userfaultfd, see:

  linux commit e346b3813067d4b17383f975f197a9aa28a3b077
  mm/mremap: add MREMAP_DONTUNMAP to mremap()

include/sys/mman.h

@@ -101,6 +101,7 @@ extern "C" {
 #ifdef _GNU_SOURCE
 #define MREMAP_MAYMOVE 1
 #define MREMAP_FIXED 2
+#define MREMAP_DONTUNMAP 4
 
 #define MLOCK_ONFAULT 0x01