Make lint(1) a cross-tool.

Make lint(1) a cross-tool.

(See commit log for usr.bin/xlint/Makefile,v 1.11 for what was wrong
with enabling build of lint libraries in rev. 1.12.)

This fixes cross-arch compiles (running binaries for a different arch
when generating lint.7 and lint libraries) and cross-branch compiles
(4.x -> 5.0 buildworld should be working again).