mk: fix unnecessary library relinking with incremental builds

Initial libs such as csu are always built (.PHONY), and their
installation to WORLDTMP was causing all the subsequent libraries to
be considered out-of-date even when in reality they were not. Use
install -C more consistently everywhere to avoid unnecessarily
updating the mtimes in WORLDTMP, fixing this problem.

This cut down my no-change buildworld time from 30 to 15 seconds.

Fixes: https://lists.freebsd.org/pipermail/freebsd-current/2016-May/061481.html
TODO.1

Reviewed by: sjg
Sponsored by: https://www.patreon.com/valpackett
Differential Revision: https://reviews.freebsd.org/D39980