SIGTRAP/SIGILL are used on ppc/aarch64 to implement traps in compiled code (print a trap message and stop with a fatal error).
JDK-8255711 reworked and unified that coding. Calls down into VMError::report_and_die() had been factored out of platform dependent code. Since that change, traps are handled the same as normal crashes, so we loose the detail message associated with a trap. This functionality should be restored.