Gdb: Add "osv runqueue" command

This patch adds a "osv runqueue" command to gdb, which shows the (sorted)
run queue of each CPU. Note that the run queue does not include the thread
currently running - just those which are ready to run.

This command is useful for debugging the scheduler and load balancer, but
is less generally useful than the "osv info threads" command, which lists
all threads, specially marking those which are runnable as well as those
which are running.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>