Reduce the disc1.iso image size by setting WITHOUT_CLANG=1 for the

Reduce the disc1.iso image size by setting WITHOUT_CLANG=1 for the
'system' target.  Since clang is not enabled by default for releng/9.2,
this has no user visible changes by default.

This brings 9.2-R* well enough below the 700MB mark so that we do not
need to be concerned with it fitting on standard CD-ROM medium.

This is a direct commit to releng/9.2, pending further review for head/
and stable/9/.

Discussed with:	delphij, hrs
Approved by:	re (marius)
Sponsored by:	The FreeBSD Foundation

git-svn-id: svn://svn.freebsd.org/base/releng/9.2@255209 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f