Optimize r270798, only do the second pass over non-plt relocations
when the first pass found IFUNCs.

Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270802 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Document the whole settings needed to build a debug version of rtld.

Sponsored by:	The FreeBSD Foundation
MFC after:	3 days


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270803 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f