Makefile: Move specific targets to the bottom

The Makefile is already a mess. Keep generic rules together at the
top, and target-specific variable settings and rules at the bottom.
open_noatime: Tentative O_NOATIME

This is a simple shared library which wraps calls to open() and
openat(), always trying to open the file with O_NOATIME. If that fails
with EPERM, the system call is tried again without O_NOATIME. This is
useful for LD_PRELOAD'ing in backup programs, if for some reason one
doesn't want to or can't add noatime to the mount options for /home.