Makefile: Use C-based 'which' on Tiger.

Unlike the 10.5+ 'which' command, which is a C program, the
OS-supplied 10.4 version is a csh script, with some deficiencies.
Since 2019, legacy-support has been supplying a patched version
of the csh script correcting some, but not all, deficiencies.  In
particular, the replacement version doesn't implement the '-a'
option, without providing an informative error message.

This change replaces the patched csh script with a build of the
10.5 version.  For consistency, it has the same universality as
the Apple-supplied programs, regardless of whether legacy-support
is being built universal in general.  Specifically, it uses the
new binarchs tool to copy the architectures from /usr/bin/true,
since full universal builds don't work on 10.4 ppc.

TESTED:
Built and tested with all option combinations.
Tested the updated install target.