ncurses: avoid hardcoded assumptions about the layout of .OBJDIR

Abstract out the details of the FreeBSD build into a $TINFO_OBJDIR that
external builds can override if they orchestrate the build a bit
differently and have a different objdir layout as a result. This makes
the ncurses build a little bit more flexible without requiring weird
backflips.

Reviewed by: bapt, sjg
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D41834