add memfd_create syscall wrapper

memfd_create was added in linux v3.17 and glibc has api for it.

src/linux/memfd_create.c

@@ -0,0 +1,8 @@
+#define _GNU_SOURCE 1
+#include <sys/mman.h>
+#include "syscall.h"
+
+int memfd_create(const char *name, unsigned flags)
+{
+	return syscall(SYS_memfd_create, name, flags);
+}