Forward abort calls to Unikraft's UK_CRASH

Forward abort calls to Unikraft's UK_CRASH

The signals are currently ignored and that causes musl to `exit` the
thread, which doesn't cause any output. Therefore, it's very unclear
what happened. By calling `UK_CRASH` there is a clear indication that
something went wrong.

Signed-off-by: Marco Schlumpp <marco@unikraft.io>
Reviewed-by: Eduard Vintilă <eduard.vintila47@gmail.com>
Reviewed-by: Sergiu Moga <sergiu.moga@protonmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #41