lldb: dereference stoppcbs based on target kernel version

As of 9fb6718d1b18 / __FreeBSD_version 1400089 stoppcbs is a pointer to
the pcbs array. Dereference stoppcbs on sufficiently new kernels.

Reviewed by: markj (earlier)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39939