assimp: disable on Microblaze

This package triggers an infinite loop bug in gcc on the Microblaze
architecture when the optimization level is O1, O2 or O3. This bug has
been reported at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71124.

While Buildroot by default uses an Os optimization level, assimp's build
system overrides that by O3 by default.

This problem is causing timeouts in the autobuilders that make them
consume 100% of CPU during 8 hours (the timeout used by the autobuilder
scripts).

Fixes:

  http://autobuild.buildroot.net/results/084fc537ab81aed278126f173daf99f2699ef22c/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>