package/brotli: work around 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 brotli 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.net/results/d86/d86251974a0a348a64d9a1d1fd7d02dd4aff0792/



Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>