When we get a cache hit for a stage1 compilation, we need to know about
some of the flags such as have_winmain or have_dllmain to know which
subsystem to infer during linking.
To do this, we append a hex-encoded byte into the intentionally-dangling
symlink which contains the cache hash digest rather than a filename. The
hex-encoded byte contains the flags we need to infer the subsystem
during linking.