freebsd-update: Library ordering

Upgrading from 14.x to 15.x with freebsd-update broke because libc
depends on the new libsys library; freebsd-update installed the new
libc before creating libsys, and every step after that failed because
all the tools (including gunzip and install) are dynamically linked
and need a working libc.

Enforce ordering when installing shared objects: First libsys, then
libc, then libthr, and then all the rest of the shared object files.

This is a candidate for an Errata Notice since the issue this fixes
breaks upgrades.

PR: 289769
Reported by: Graham Perrin
Reviewed by: kib
MFC after: 3 days
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D52688

(cherry picked from commit 7ece602e00e85195fc426a2401c49921cd39735e)