Use sig_atomic_t for the late SIGXCPU test state
Merged
XA
5d ago+5+5+0123456789-4-4-0123456789
Intent: make the values that the SIGXCPU handler in tests/test-rlimit-cpu leaves for the test determinate after it returns.
This PR: late_seen and late_n become volatile sig_atomic_t, which C11 5.1.2.3p5 requires for an object a handler modifies. Follow-up to cubic's comment on #475, which merged before this fix was pushed.
make check passes; test-rlimit-cpu passes 11/11 under elfuse.