Look for correct "ar" to cross-compile

For some reason, autoconf doesn't by default have an "AC_PROG_AR" for
this -- possibly it's assumed that any "ar" you have will work
everyplace.

Fixes bug 3909; found by sid77.

This fixes a build issue first present in fdbdb4dc, but the bug
(of not using a correct ar) has been in every Tor version ever: it
just didn't matter until then.