package/lmbench: fix build failure due to gcc bug 101915

package/lmbench: fix build failure due to gcc bug 101915

The lmbench package exhibits gcc bug 101915 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_101915=y.

Fixes:
http://autobuild.buildroot.net/results/ae1/ae1e4d61ed367c6cb64442c60d98882cc7985346/

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