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

kernel/src/Api/Syscall.cpp

@@ -400,6 +400,9 @@ namespace montauk::abi {
                 return (int64_t)Sys_ProcList((ProcInfo*)frame->arg1, (int)frame->arg2);
             case SYS_KILL:
                 return (int64_t)Sys_Kill((int)frame->arg1);
+            case SYS_ABORT:
+                Sys_Abort();
+                return Success;
             case SYS_SETSESSION:
                 return (int64_t)Sys_SetSession();
             case SYS_KILLSESSION: