Remove real time signals references

Remove real time signals references

At this moment uksignal doesn't support real time
signals (signal codes greater than 31). However, the Go runtime
assumes that it does and it tries to init signals up to SIGRTMAX,
causing any go application to fail. This commit avoids this situation.
These changes should be removed when real time signals support will
be added to uksignal.

Signed-off-by: Răzvan Vîrtan <virtanrazvan@gmail.com>
Reviewed-by: Razvan Deaconescu <razvan.deaconescu@cs.pub.ro>
Approved-by: Razvan Deaconescu <razvan.deaconescu@cs.pub.ro>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Pull-Request: #4