Export the single step function from the AArch64 simulator.

	* interp.c (sim_create_inferior): Allow for being called with a
	NULL abfd parameter.  If a bfd is provided, initialise the sim
	with that start address.
	* simulator.c (HALT_NYI): Just print out the numeric value of the
	instruction when not tracing.
	(aarch64_step): Change from static to global.
	* simulator.h: Add a prototype for aarch64_step().