generate: treat rt_sigreturn as noreturn (suppress dead exit handler)

rt_sigreturn(2) restores the pre-signal execution context off the signal
stack frame and resumes the interrupted instruction; it never returns to
the instruction after the syscall. man sigreturn(2) states plainly that
"sigreturn() never returns", and tracing against /sys/kernel/tracing
confirms it: sys_enter_rt_sigreturn fires once per signal-handler return
while sys_exit_rt_sigreturn never fires.

The generator previously emitted a dead handle_sys_exit_rt_sigreturn (it
can never run) and recorded a per-tid syscall_enter_state_map entry on the
enter path that nothing would ever delete (no exit fires), leaking entries
in the bounded map on every signal-handler return.

Add rt_sigreturn to noreturnSyscalls so codegen suppresses the dead exit
handler and routes the enter handler through ior_on_noreturn_syscall_enter
(sampling decision only, no map write), exactly like exit/exit_group. The
enter null_event is still emitted, and the FamilySignals/KindNull
classification is unchanged. Regenerated the C/Go artifacts and the result
baseline accordingly, and generalized the related comments.

Lock-in tests: TestRtSigreturnIsNoreturn asserts rt_sigreturn is noreturn;
TestRtSigSiblingsAreNotNoreturn guards that the returning rt_sig* siblings
are not; TestGenerateExitNoreturnHandlers now also covers rt_sigreturn.

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

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