Implement single stepping on arm64. We need to set the single step bits in

Implement single stepping on arm64. We need to set the single step bits in
the processor and debug state registers. A flag has been added to the pcb
to tell us when to enable single stepping for a given thread.

Reviewed by:	kib
Sponsored by:	ABT Systems Ltd
Differential Revision:	https://reviews.freebsd.org/D4730