package/python-brotli: fix build failure due to gcc bug 68485

The python-brotli package exhibits gcc bug 68485 when built for the
Microblaze architecture with optimization enabled, which causes a build
failure.

As done for other packages in Buildroot work around this gcc bug by
setting optimization to -O0 if BR2_TOOLCHAIN_HAS_GCC_BUG_68485=y.

Fixes:

http://autobuild.buildroot.net/results/24b/24b23175ab27615fb377bb4d5f6c656dccf10a86/



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