Off-chip emulation with pthreads causing "could not find signal in global observer"

[andersondomingues]

Off-chip module side thread keeps listening for UDP packets when it should be killed, causing "could not find signal in global observer" messages to be spammed in the reporting output. Packet sending is not affected.

Steps for reproducing the bug:

1. start any simulation
2. press CTRL+C once to stop the engine (do not press multiple times, wait for the simulation to finish properly)
3. see a couple of messages at the end of simulation regarding the orphaned thread

Solution: replace the pthread implementation with the asynchornous I/O impl. used in GDBRSP server.