package/protobuf: work around gcc bug 85180

package/protobuf: work around gcc bug 85180

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

Fixes:
http://autobuild.buildroot.net/results/73dc9610a13d6e14eec58d529617210d93d5dec4/

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
[Arnout: fix variable name]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>