1.0.5.18: trapping-based stepper on the Sparc

1.0.5.18: trapping-based stepper on the Sparc
	... implement instrumentation for :before and :around cases;
	... as suggested by Juho, use NTH-INTERRUPT-CONTEXT rather than
	allocating a SAP in a signal handler;
	... actually run the tests on sparc too.