mk: Move vm stack test debug symbols

Avoid the creation of a /usr/tests/sys/vm/stack/.debug directory by
placing sys/vm/stack test debug symbols under /usr/lib/debug.

Reviewed by: emaste
Approved by: emaste (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D48796

(cherry picked from commit 88d448ec815c1fda27be94846f88ccd96251febd)
(cherry picked from commit 955f974a1a37cf3ae8b241327b1fef74c8932855)