lib/posix-process/signal: Add default signal actions

lib/posix-process/signal: Add default signal actions

Add per-architecture default signal actions. These happen to be the
same for arm64 and x86_64, yet it's best to define them in separate
headers for forwards compatibility with future architectures.

Signed-off-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Ioan-Teodor Teugea <ioan_teodor.teugea@stud.acs.upb.ro>
Reviewed-by: Sergiu Moga <sergiu@unikraft.io>
Reviewed-by: Andrei Tatar <andrei@unikraft.io>
Approved-by: Andrei Tatar <andrei@unikraft.io>
GitHub-Closes: #1248