Disable BINUTILS by default on i386

The retirement of obsolete binutils 2.17.50 has been in progress for
quite some time. All tools other than GNU as were removed prior to this
commit, and it was built only on two archs:

i386, installed as /usr/bin/as
amd64, installed as /usr/bin/as and as a bootstrap tool

The i386 exp-run has completed and failures have been addressed in the
individual ports, so disable it there.

PR: 233611, 205250 [exp-run]
Sponsored by: The FreeBSD Foundation