package/git: work around gcc bug 85180

package/git: 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 git 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/7ad/7adff001631053ae5a3cb3e176d321f6a2d3cceb/

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>