package/opencv3: fix build due to gcc bug 68485

With Microblaze Gcc version <= 9.x the build fails due to gcc bug 68485:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68485. The bug show up when
building opencv3 with optimization but not when building with -O0. To
work around this, if BR2_TOOLCHAIN_HAS_GCC_BUG_68458=y, we force using
-O0.

Fixes:
 - http://autobuild.buildroot.org/results/c78eac84d1c5a6702e7759cd5364da1c3e399b4b



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>