package/kismet: work around gcc bug 85180

package/kismet: 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 kismet 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/02f6e616631f05a4f15f31f2b2e67d65ba7afcb0/

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>