package/collectd: work around gcc bug 68485

collectd is impacted by the microblaze-specific gcc bug #68485 [0].

As for all other impacted packages, force no optimisation when using a
toolchain riddled with that bug.

Fixes:
   http://autobuild.buildroot.net/results/6a8/6a87bbe17e6f606a97097800dc484a7b17bffa7d/

[0] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68485



Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>