Enforce -lpthread and -lc to always be the 2 last components of the link list

Differential Revision:	https://reviews.freebsd.org/D1118
Suggested by:		kib


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274849 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f