Makefile.inc1: use make builtin :ts instead of sed

Commit 3eb1b4da3cf7 replaced "xargs -n1" with a sed expression to
convert from space to newline as a list separator for *-old-* targets.
Dan Nelson followed up with a suggestion to use make's built-in :ts
instead, which should be slightly more efficient.

Reviewed by: sjg
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39972