build uses host's ctfmerge

[Timo Buhrmester <fstd.lkml%gmail.com@localhost>]

While bisecting, I found that there was a period where ctfmerge was
given - as a temporary hack - another command-line parameter `-S`,
which was removed again a while later.

Building NetBSD revisions that fall into that period, on a post-`-S`
host will fail, because the host's ctfmerge is run with -S; a usage
message is the result.

I got away by making a ctfmerge wrapper that strips any -S present,
but I'm wondering if using a host tool is correct in the first place.

Shouldn't NetBSD build its own ctfmerge and put in into the tooldir?