Fix optimized IRIX kernel module builds

Fix optimized IRIX kernel module builds

Commit 9f94892f8d996a522e7801ef6088a13769bee7c2 (from 2006)
introduced per-file CFLAGS, using $(CFLAGS-$@); this construct
is not parsed well by IRIX make, which ends up attempting to
expand '$@)' and finding mismatched parentheses.

Commit 5987e2923a2670a27a801461dc9668ec88ed7d2a (from 2007) followed,
fixing the IRIX build but only for the NOOPT case.  This left the
problematic expression in CFLAGS_OPT until 2013, when another RT
ticket was filed reporting the continued breakage.  That ticket
was then ignored until 2015 (now) with no particular cries of
outrage on the mailing lists.  Perhaps this gives some indication
of the size and/or mindset of the IRIX userbase.  (There have
been successful IRIX installations during this time period, so
presumably it was discovered that disabling optimizations helped
the build along.)

FIXES 131621

Reviewed-on: http://gerrit.openafs.org/12111
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams <3chas3@gmail.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 767694d9ec86fc9451f5a4ba2ec7405c29986a21)

Change-Id: Ie5d349b1e9f8a768efcb461d7367d2d7deac31f6
Reviewed-on: https://gerrit.openafs.org/12198
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>