Do not strip all when stripping an explicit symbol

When requested to strip specific symbols (-N flag) the default should be
to strip nothing (other than the requested symbols). This is consistent
with binutils strip(1).

PR:		196038
Reviewed by:	imp
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D1327


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275862 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f