libc: fix __unused name collision

__unused is used in libc for some unused field, and also by bsd to mark
unused arguments.  Change the libc instance to resolve include conflicts.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
rcu hashtable: add size() and empty()

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>