support posix_madvise (previous a stub)

the check against MADV_DONTNEED to because linux MADV_DONTNEED
semantics conflict dangerously with the POSIX semantics

src/mman/posix_madvise.c

@@ -1,6 +1,8 @@
+#define _GNU_SOURCE
 #include <sys/mman.h>
 
 int posix_madvise(void *addr, size_t len, int advice)
 {
-	return 0;
+	if (advice == MADV_DONTNEED) return 0;
+	return -__syscall(SYS_madvise, addr, len, advice);
 }