Add /usr/lib/debug directory to hier(7)

The canonical standalone debug directory established by the GNU
toolchain is /usr/lib/debug, and we use it when WITH_DEBUG_FILES is set.
Mention it in the file system hierarchy page.

Reviewed by:	bcr
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D1134


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274349 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f