add MADV_FREE madvise command from linux v4.5

allows the os to free the marked pages lazily on memory pressure.
expected to increase malloc performance.
new in linux commit 854e9ed09dedf0c19ac8640e91bcc74bc3f9e5c9

include/sys/mman.h

@@ -62,6 +62,7 @@ extern "C" {
 #define MADV_SEQUENTIAL  2
 #define MADV_WILLNEED    3
 #define MADV_DONTNEED    4
+#define MADV_FREE        8
 #define MADV_REMOVE      9
 #define MADV_DONTFORK    10
 #define MADV_DOFORK      11