feat: add process termination codes and SYS_ABORT; whole-process abort() from any thread in libc and montauk-rt

kernel/src/Sched/Scheduler.cpp

@@ -1885,7 +1885,7 @@ namespace Sched {
                 continue;
             }
 
-            primary.exitCode = 256 + 9; /* killed (SIGKILL) */
+            primary.exitCode = TerminatedKilled;
             primary.killPending = true;
             if (primary.state == ProcessState::Blocked) {
                 primary.state = ProcessState::Ready;