package/chocolate-doom: work around gcc bug 85180

With Microblaze Gcc version < 8.x the build hangs due to bug 85180:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85180. The bug shows up
when building chocolate-doom with optimization but not when building
with -O0. To work around this bug, if
BR2_TOOLCHAIN_HAS_GCC_BUG_85180=y, we force using -O0.

Fixes:

  http://autobuild.buildroot.net/results/e570839576be8963dc6bd36342e2f857da3c6146/



Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
[Thomas: tweak CFLAGS logic, improve commit log]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>