devel/gdb: kgdb: Handle stoppcbs compat

Use __FreeBSD_version to decide whether to dereference the stoppcbs
symbol. PORTREVISION will be bumped as part of the next commit, which
fixes another KBI breakage.

Reviewed by: jhb, kevans
Approved by: jhb (maintainer)

(cherry picked from commit 7e2852846872215c7d7ad36d9ea15d593bbf343e)