codegen: suppress unreachable sys_exit handlers for noreturn syscalls

exit and exit_group never return to userspace, so their sys_exit
tracepoints can never fire. The generator previously emitted matching
EXIT_RET_EVENT handlers anyway, producing dead code in the generated BPF
program. classifySyscall now skips exit-handler emission for noreturn
syscalls via isNoreturnSyscall, and the regenerated artifacts drop the
sys_exit_exit / sys_exit_exit_group handlers (enter handlers are kept).

Tests updated to match the new reality:
- TestGenerateExitNoreturnHandlers asserts no exit handler is emitted.
- TestClassifySyscallPairEmitsAllFamilies exempts noreturn syscalls from
  the exit-handler-required assertion while staying strict for all others.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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