Invoke objcopy on the right object when building Scrt1.o on i386.

This was a copy-paste bug in r362902. While here, switch to using
${.TARGET}.

Reported by: Kjell Tore Ullavik <ktullavik@gmail.com>
Reviewed by: kib
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D25585