Fix MMU cache statistics signal handling

[jserv]

This restores MMU cache statistics functionality broken by coroutine refactoring. The refactor introduced separate execution paths (SMP mode, single-hart mode, debug mode) but signal handling was only implemented in the SMP path.

Signal handling follows async-signal-safe pattern: handler only sets a volatile sig_atomic_t flag, with stats printing deferred to main loop context. All execution paths now properly clean up resources (pfds, file descriptors) before exit.

Close #113

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Summary by cubic

Fixes MMU cache statistics when the emulator is interrupted. Signals are now handled consistently across SMP, single-hart, and debug paths, and stats print once after a clean shutdown.

• Bug Fixes
  • Use an async-signal-safe handler that only sets a volatile sig_atomic_t flag.
  • Check the flag in SMP, single-hart, and GDB loops; break to allow cleanup.
  • Defer stats printing to the end of main after closing pfds/file descriptors.

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